Based on my personal experience: three months if you know nothing about programming at the outset. Here’s why:
The goal was to write and deploy a full-fledged ERP for my company in 90 days using Ruby on Rails. The app is now live and the whole company is using it! Here is where our time was spent:
2011-09-01 to 2013-04-01 (19 months) – Using and getting frustrated enough with our old ERP to start thinking about learning to program and writing a new one:
2013-04-01 to 2013-08-01 (4 months) – Learning Microsoft Access and building a version of the new ERP in it, before I ever heard about Ruby on Rails
2013-08-01 to 2013-11-01 (3 months) – Learning basic Unix and Rails. Deciding whether these were the right tools instead of Microsoft Access
2013-11-01 (start of the 90-day countdown) to 2014-06-01 (7 months – only 140 days past our 90 day deadline!!!) – Actually writing the app
The rest of history – Maintaining the app
So, in three months if you dedicate most of your spare time to it, you can go from knowing absolutely nothing about computers, to being able to build Ruby on Rails apps that have real business uses. If that interests you, check out my list of suggested resources to do so.