Some things I was able to finish in about a week but they never saw the light of day because they didn't pass my personal quality testing. I mean, if I don't like it, and I made it,  no one will, right? Others get fairly well along but then a problem will arise which will cause me to either start over from scratch or force me to reconsider some aspect of what I've done. The 3 products I have selling, one took me about a week if you were to tally the hours (I have a day gig so that slows things down). Another, took me about a month (again, this is total hours worked). The last one was finished (rigged by PhilC no less) but then scrapped due to my personal quality testing and decision to go for more realism in my imagined creature design. The remake took me about a month and a half to complete (total time worked) including an entirely new rig (which was a real shame because Phil did awesome work) and learning a new program to texture with.

So it depends on you, how well you know what you're doing, what your sense of quality is for the item in question, and the amount of time you have in a day to work on it.