Agree with Barcode... there's a million ways to answer that question.  It depends on a lot of variables, such as skill and experience as you mentioned, which modelling program you're coming from (some import into poser with dreamlike ease, while others take a bunch of fiddling and tweaking to get it to work right), what you're making (I made a fliptop lighter, with rigging and pretty renders in under a day, but there's a clothes outfit that I've been working on for weeks).  And lets not even get into waiting for the Renderosity testing queue or having to have the item beta tested if it fails the acid test.

That's for models you've made yourself... for textures for other's products, there's a whole other process.  Getting the right seam guides, re-mapping the UVs if necessary, checking for stretching on the textures, and again if you fail the Renderosity upload testing, then you have to turn around, find beta testers, etc. etc.  All that eats into your time, from project beginning to being available in the marketplace.