I'm setting aside the question of whether or why to switch. I'm also setting aside the possibility of starting to learn now on Python 2.7, which is what I think you should really do assuming you don't intend to procrastinate it ;)

Assuming you do want to wait on Python 3 adoption, your timing should depend on the framework you want to use, because effectively each one has its own community and ecosystem, and their adoption is at completely different rates.

If Pyramid looks good to you, for example, it already is on board with Python 3. Bottle is on board with Python 3. If you want to use Django, which is what most people will want to do, you should just wait on Django to release a Python 3 version, and Django should be usable on Python 3 within the year. If you want to use Flask (considered the closest analogue of Ruby's Sinatra) then it could take a while.