How Long Did Islam Survive Among Slaves in the American South and How Did it Impact African American Culture?

I will focus on the lasting impacts on culture. Islam has had a longer and more visible impact on Africana culture in the Americas outside of the United States. But regarding your question as referring to African-American culture, I will focus on one well-researched connection between Muslim slaves and African-American culture: music.

Per the Routledge Handbook of Islam in the West, there were two trends of music practiced by enslaved people in the US:

non-Muslim slave groups from coastal West Africa and Central Africa, who relied heavily on (rapid) drumming, polyrhythm, call and response, group singing, and short melodic lines and chants for their musical expression
slaves from Sahel/Senegambia with a traditional emphasis on string and wind instruments, with a solo, moaning kind of song "that blues expert Alan Lomax called a 'high lonesome complaint.'"

To quote the Handbook:

Due to Southern plantation owners’ fear of slave revolt and uprising, drumming and group chants were outlawed, while Sahelian slaves were able to adapt their skills to local instruments such as the fiddle or guitar, later even producing the banjo as an American incarnation of their traditional lute. As a result of the seemingly less threatening nature of their style, they were allowed to perform their music, sometimes even at slaveholder’s balls, which allowed for the music’s migration across the Deep South, including Mississippi, the birthplace of the blues.

Gerhard Kubik's Africa and the Blues provides an exhaustive look at blues' African connections. He describes these two distinct musical styles from the west central Sudanic Belt as such:

(I) A strongly Arabic-Islamic song style, as found for example among the Hausa. It is characterized by melisma [changing notes while singing the same syllable], wavy intonation, pitch instabilities within a pentatonic framework, and a declamatory voice production. All this behavior develops over a central reference tone, sometimes like a bourdon. (2) An ancient west central Sudanic stratum of pentatonic song composition, often associated with simple work rhythms in a regular meter, but with notable off-beat accents. This style reaches back perhaps thousands of years to the early West African sorghum agriculturalists, now scattered through the Sudanic Belt in remote savanna, often mountainous areas. This style has remained unaffected by the Arabic/Islamic musical intrusion which reached West Africa along the trans-Saharan trading routes, and subsequently spread from the early Islamic states, Mali (ca. 1230–1400 A.D.) and Songhai (ca. 1464-1600 A.D.) to the emerging Hausa city states and Fulbe courts.

There are also musical differences between these styles: the Islamic song style is originally urban, is cosmopolitan, reflects social stratification, incorporates many bardic genres, and usually uses instruments from North Africa. The "ancient Nigritic" (as he calls is) style is "rural, is a part of the millet-agricultural life cycle, and, if accompanied at all, makes use of percussive devices that have a millenia-old history in the savanna."

To illustrate both the differences and their connections to blues music, Kubik's book fortunately has an accompanying album (you can listen to the full album here). To give an example of pre-Islamic African musical style and the blues, Kubik compares this song sung by a Tikar woman while grinding corn with Mississippi Matilda's performance of Hard Working Woman. Kubik gives a long musical analysis, but to give snippets:

the song style of this [Tikar] woman represents an older, pre-Islamic West African tonality. There is no melisma. Its basic outline is in disjunct intervals, with falling melodic ductus of each line. The overall impression of the melody stunningly reminds one of the blues. In rhythmic organization her cycle of actions on the grinding stone covers 36 elementary pulses (4 times 9) for each line of the song. There are strong off-beat accents in the scraper-like grinding rhythm. These accents are placed so as to produce a swinging triple rhythm...Most characteristic also in the Tikar woman’s song is the fact that the singer always returns to a basic tonal center at the end of each line, just as in most blues.
Mississippi Matilda’s “Hard Working Woman” (1936) can be compared to the Tikar woman’s performance, not only because of the topic of doing hard work, but also because the voice timbre of the two women is similar. By chance, Mississippi Matilda even starts her song with a phrase close in melodic ductus to that of the Tikar woman (see my transcription of her melodic line, Example 9), although she goes on to develop it in a different way.
Matilda’s is a type of blues in a somewhat popularized idiom, suggested largely by the two-guitar accompaniment. It is this quality that provides perhaps all the essential stylistic differences between “Hard Working Woman” and the female Tikar grinding song. If we can abstract Mississippi Matilda’s vocal line and its adjustments from the guitar accompaniment, then the parallels between the two female singers can hardly be overlooked.

To contrast, we can look at an Islamic influence in Kubik's comparison of Hausa musician Meigogué's untitled song with one-string fiddle to Big Joe Williams' performance of Stack o'Dollars. To quote Kubik's analysis:

Meigogué’s tonal material for the voice and his instrument is pentatonic, but in his shrill-timbred singing style he uses melisma extensively, including some microtonal shifts of the voice from the pentatonic skeleton, and lots of glissandi. Like the blues of Big Joe Williams, Meigogué’s music is also based on the melodic circumlocation of a central tone that functions as a reference, sometimes even like a bourdon. Voice and fiddle part alternate, establishing a slightly overlapping responsorial structure; the fiddle part begins before the voice part has ended.

Meigogué’s Hausa singing style is, of course, highly determined by the centuries-old cultural contact with the Arabic/Islamic world. The presence of the one-stringed gogé, with its history going back to the Maghreb (North Africa), is another testimony to these contacts. While the basic scalar framework used is pentatonic, the intonation of many tones both in the voice and the fiddle parts is wavy, bent, often approaching a tone from below before reaching maximum height, then quickly collapsing.

Side Note:

While there is utility in separating out the Islamic and pre-Islamic styles of music, this isn't to suggest that there was a sharp segregation of division between Islam and pre-Islamic religion in Africa. As stated above, these two styles were blended in African-American music. Additionally, the history of Islam in Africa illustrates a history of blending and mixing of religious practices. As with the syncretic history of Christianity in Europe that has given us "pagan" Christian holidays, rituals, and culture, Africa has a similar history with Islam. Non-Muslims adopted Islamic practices without officially converting (such as using pieces of the Quran as talismans), and Muslims reconciled Islam with their previous culture. An example of the latter can be seen in the Islamic Mali Empire's famous Epic of Sundiata, which tells an Islamicized narrative about the first Malian Emperor (who was probably not even a Muslim).

Additionally, the notion that slavers found Islamic musical style to be less threatening does not mean that Islam or Muslim slaves were not threatening to slavery. To limit us to Haiti, there is evidence to suggest that key figures in the Revolution, such as François Mackandal or Dutty Boukman may have been Muslims. As historian Sylviane Diouf states:

the Muslims in general, played a role in the Haitian revolts and ultimately in the Haitian Revolution through their occult skills, literacy, and military traditions. The marabouts [Muslim religious leaders] provided protections to the insurgents in the form of amulets, and the Muslims used Arabic to communicate during uprisings, as Colonel Malenfant recorded. Though their role and contribution have not been acknowledged, the Muslims were part of the success of the Haitian Revolution.

Back to Islam in African-American music: As alluded to above, while enslaved people from Islamic Africa were outnumbered by those from non-Islamic Africa, drums were outlawed in the South following the Kongo uprising of 1739 in Stono, South Carolina. Thus, musicians who played stringed and wind instruments were not just at an advantage, but were able to adapt to fiddles and guitars and be used by slaveowners "in their own balls, so they could continue to exercise their talents openly." These musicians were exempt from field working and thus had time and the instruments to develop their skills. It's also important to note that the Western instruments they adapted to were not strict European inventions that came from a vacuum. The banjo was developed via influence from African stringed gourd instruments, and other Western stringed instruments (violin, guitar) were also influenced by Islamic stringed instruments from West Asia and North Africa.

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