Are you new to programming? Python? Or just new to Web2py? I had a very modest amount of prior programming experience, none of it on Python, and I was able to make very good progress on a storefront application in a matter of days and weeks. I suspect adding the final 20% of polish will take 80% of the time, though.
I looked at a variety of frameworks (CodeIgniter, Rails, Django) and found Web2py to be both the easiest to lear and use and also offer the most productivity. As Villas notes, the front-end can take the most energy depending on how fancy you want to get with your HTML, JavaScript, Ajax, etc. I usually suggest keeping it simple. You can layer in complexity over time.
