Paul, for me it depends on how many projects are on going at any one time. I always try to have something that needs painting on the bench and may be a couple of figures I'm putting together. More than one sculpting project allows me to swap to the next project when I am leaving the putty on the first to dry. Currently I have five projects, some are slow to progress and some seem to hog my time and get done fairly quickly. On average I'd say a month to a month and a half per project, as have the time on my hands, Granddaughter/wife/other children permitting, it doesn't cause too many delays.
That said I had a disaster with my Tambour Major today but I will explain that on my thread.
Cheers Simon