Either way (immediately or in a few months) DON'T renew your contract with Sprint, LOL.
If you are new to (and will continue to) following phone releases, you will be very disappointed when your waiting, and waiting, and waiting, for Sprint to finally get that hot new phone.
To answer your question though: It varies. As it was said, Apple can do it immediately. But since this is Google doing the release, WHO KNOWS? Optomistic though that it will be sooner than later since peeps have already had the phone for a month+