Each painting has so many variables. Pose, lighting, background, patterns on clothing, all of these things play a part. I do have ballpark figures I use to estimate if I can complete a particular painting within a requested timeframe, but I purposely keep these loose. The painting process can include any number of surprises.
Most of the very realistic work I do takes weeks and months to complete as I work in layers and so paint needs to dry before I can proceed. And surprisingly, I have found that a smaller canvas is not necessarily easier or faster to produce.
Typically it takes me between 2 and 6 months to complete a portrait, and I know when a portrait is complete not by the number of hours I've put into it, but by some internal signal that says, "That's it! Put the paintbrush down and walk away!"