There's really no "specific" time line, it all depends on you! If you feel like you are at the point in your life with your situation and your current dog that you can handle it, by all means go ahead!
The only "NO'S" I've ever heard is getting two PUPPIES at the same time, because it is really hard trying to go through obedience with both. However I've known several people to do it successfully.
I think I'll be getting another one once my six month old is about the age of yours