this site is great for exploring questions like yours. It estimates a trip from Rome to Alexandria would take about 15 days if you traveled over open sea, or about 26 days if you took the (safer) coastal route.

Orbis built by Stanford can give you very specific information about this. It's sort of the Google maps of ancient (and probably medieval) travel. It uses known maritime locations that were navigable by the appropriate technologies of the time. It does assume that seafaring conditions will be consistent throughout the year built do include random fluctuations that make crossing more time consuming.

From some further research I have found out that this model is based on ship propelled by sail rather than oar, which may not be what you're after but the notes provided indicate this would have been the principle method of transportation at the time.