An easier question to answer is "when is it OK to start farting around a new girlfriend?" Once you get that one sorted out then, move on to bigger concerns like when to propose.
Always do a prenup too. Do it at the beginning when you are both on good terms and it protects both of you (who knows these days, she could be the one that becomes a multi-millionaire). If you never get divorced then, it's just a worthless piece of paper that ended up meaning nothing. You just don't want to figure out how to settle a divorce if you end up hating each others guts and if she won't agree to a prenup, that tells you something also.