Every 3 months is when you should replace your toothbrush with a new one and that's what I go by.
After 3 months I get a new toothbrush and throw my old one out.
You can go as long as waiting 4 to 5 months before replacing the toothbrush but to me that's too long and you can get toothbrushes for very cheap so it's not worth keeping and older toothbrush around any longer than you need too.