I'll try to answer your question, sorry if it doesn't satisfy you, especially since afaik to this day there’s no single dedicated study or research geared towards the daily life of the janissaries.

First, in the 15th century Murad I has laid down the rules for the janissaries: Total obedience to officers, strict military behaviour, unity of purpose, no extremes of luxury or abstinence, strict piety, capital punishments of a distinctive sort, punishment by only their officers, promotion by seniority, looking after their own dependants, no beards for ordinary soldiers, no marriage until retirement, living only in barracks, no other trades, fulltime military training, and no alcohol or gambling. But of course, this was just only rules and isn’t really enforced as time goes. Also, a disclaimer: though the janissaries are technically slaves of the sultan, they were not slaves as understood by Western conception. In any case, the janissary’s status made the elite members of the Ottoman society. Because of their status, janissaries gained prestige, social connections, tax-exemption, (meager) salary, accommodations, and definitely ability to do whatever an Ottoman elite (or commoner) could do. Because of that, we can’t directly compare them to ordinary slaves. What I’ll write is the daily life of the janissaries particularly in peacetime, which is probably the meat of your question.

The janissaries lived in barracks which was built during the reign of Mehmed II: Eski Odalar and Yeni Odalar (Old and new barracks respectively). Yeni Odalar contained some 368 rooms, 130 arbors, 90 exercise grounds, 20 pavilions, 4 tekkes lodges, and 158 stables. Every room had a kitchen, storage, laundry room, ward, bench, and an arbor. Even though the janissaries are supposed to live only in the barracks, entering the 17th century with the civilianization of the janissaries, janissaries often owned or rented houses within Istanbul, married and settled down. In peacetime, janissaries are tasked with non-combatant roles, such as being night watchmen, firemen, policemen, and spies. Each janissary battalion was stationed in one of Istanbul’s numerous districts, where it operated from kolluk, which functioned as a modern-day police station. The janissaries also played an important role in Ottoman ceremonies and rituals, for example, janissary paydays often timed to coincide with the presence in the capital of foreign diplomatic missions. On these occasions, the presence of the janissary regiments in full strength made a powerful impression on Western envoys. Though the janissaries had drill and shooting practices, these exercises aren’t regular and by the 18th century few attend such trainings anymore.

Pay for the janissaries are low, in the 17th century it’s around 8 to 15 akçes per day. On average, a rank-and-file janissary would earn 9 akçes per day. In comparison, an unskilled worker would earn around 19 akçes per day. Some 9% of the janissaries actually lived under poverty line, possessing less than 1,000 akçes. The poorest of them doesn’t even own anything except clothes and basic household items. This, exacerbated by rising costs due to wars and inflation forced janissaries to make ends meet by working or entering trades. Now, janissary entrepreneurialism isn’t unknown, since there are records of janissaries owning or renting shops from the 15th century, but by the 17th century most janissaries are working in one trade or another. Some become merchants, creating a complex networks of janissary mercantile activities and becoming suppliers of goods and food. Janissary merchants also were well established on the Istanbul market, becoming textile merchants, fruit and vegetable sellers, grain wholesalers, meat sellers, and other major or minor traders provisioning Istanbul. Many janissaries also entered guilds and become artisans, like butchers, cooks, tanners, boza-makers, candle-makers, carpenters, bakers, greengrocers, pastry shop owners, coffeehouse owners, tinsmiths, locksmiths, shoemakers, masons, barbers, and many other fields. Additionally, some wealthier janissaries becomes lenders and investors. For example, a janissary named Ali Beşe from Istanbul had his partnership with a certain Poulēmenos. Ali Beşe gave him 80,000 akçes, most likely to trade with. They agreed to check their accounts every March, to split the profits equally, and that Ali Beşe could withdraw his money at any time. Speaking of lending, janissary regiment lending also used regiment waqfs for their advantage. So every member of the regiment had to give a certain percentage of his salary to the waqf of his regiment. This money, deducted regularly as suggested in the janissary regulations, was used to support those in need, such as the families of deceased janissaries. During Ramadan, wealthier janissaries would donate food, candles, and firewood to the regiments to support the poorer ones. This solidarity was also maintained through cash donations to the regiment waqfs. These waqfs was to help janissaries, janissary orphans in need, and other benevolent activities, however, they also could be used as cash waqfs to accumulate cash through interest. This is how it works, the endowed capital of the waqf was given to borrowers for a certain period of time and paid back to the waqf with an extra amount, that was not explicitly called interest (rıbh), but was rather referred to as çuha bahası (cloth money) or kul akçesi (money for the kul), due to the legal concerns of interests.

To relax themselves, janissaries visited taverns and coffeehouses. Janissaries usually attend coffeehouses owned by one of their members, such janissary coffeehouses often situated next to a kolluk, and sometimes an existing coffeehouse would even double as a kolluk. Janissary coffee shops in Istanbul varied in size. Some were so luxurious, opulently furnished and decorated that they became the subject of poetic praise. They were generally established in places that had the best views and many of them were built on the city walls overlooking the sea or on columns erected in the sea. Sometimes one regiment (orta) might own a few cafes, or one janissary officer might have more than one coffeehouses. What marked Janissary coffeehouses was the insignia (nişân) of the orta, which would be placed above both the entrance to the coffee house and the hearth. Janissary coffeehouse customers were usually Janissaries of various ranks and their associates. However, non-janissaries also visited these cafes and, in fact, the owner sometimes made them visit so that the former’s income might increase. The coffeehouses offered an appropriate place in which the regiment solidarity could survive and flourish, and provided an adaptable place in which to house and absorb multiple activities. Janissary coffeehouses, just like other coffeehouses provided social, literary, and musical exchanges, to the point that there’s a term for janissary poets: aşık. Janissary coffeehouses also becomes a place to celebrate the Bektashi Sufi order followed by the janissaries. Bektashi hymns, were often sung, and on the walls there were framed inscriptions of Bektashi sayings or prayers. The opening of a Janissary coffee house was also sometimes marked by a Bektashi parade. Thus, the old connection between the Bektashi order and Janissary corps was retained and carried over to the coffee house, which became a site where Bektashism not only continued to be practiced, but actually thrived. And of course, janissary coffeehouses becomes he place where janissaries shared gossips and news about the government, and plotting political movements to preserve their rights and influence. The strategic location of janissary cafes near kolluks also led it to become a business office, where the janissaries could do their mercantile role outside of kolluks. Janissary coffeehouses could also become a place of crime however. Janissary mafias known as zorbas centered their “business” and activities in janissary coffeehouses owned by members of their gang. These zorbas perform crimes from extortion, monopolies, racketeering, seizures, and gang wars.

Sources:

The Economic and Social roles of Janissaries in a Seventeenth Century Ottoman City, The Case of Istanbul by Gülay Yılmaz

"When Grocers, Porters and other Riff-raff Become Soldiers”: Janissary Artisans and Laborers in the Nineteenth-Century Istanbul and Edirne by Mehmet Mert Sunar

Ottoman Tulips, Ottoman Coffee edited by Dana Sajdi

Bread from the Lion’s Mouth edited by Suraiya Faroqhi