Honestly it's hard to say on how long does it take to make an art for commission since there's a LOT of things that can factor in but that said, it's also depend on the buyer's patient. So it's hard to say really. That said I'm more of a "take your time" type of person.
Agreed it does from person to person and I consider myself the more patient type I do tend to get edgy if I commission something from someone and it's been over 3 months without any notice about it. I can empathize though because I can take any where from a a day to a couple months for a commission. I try to them within a month of receiving them, but sometimes I have no time. or sometimes I am not inspired to do a piece because it could turn out poorly if I force myself to work on it at times.