I don't think this is a question to be based on "length of time" in every case. True in time, things should grow to a point where this becomes something you're ready for but I think it's more about where the relationship is, the trust, the level of commitment etc. With that in mind I'm not so sure anyone here can definitively answer this for you. When you are comfortable, IMO, you will be ready and you won't need to ask others about if it's right yet. But that's my opinion. I could be wrong.