The best answer anyone can give is: whenever you feel you need one. Playing a 5 or a 6 just to play one is a bit of a waste if you don't have a direct benefit in mind from playing on one. If you think that you'd like having access to the extra low notes, like the tighter fret spacing and range across the neck by playing at the fifth fret, or like having the higher string to help with chording then a 5 or 6 would be a great purchase. Buying one just because you feel like you've "leveled up" to that point is a bit of a beginner thought process, one that I admit that I had when I was first starting out. I got my 5 and started overusing the low notes because they were there. If you feel you have a reason to have one, buy one today, if you can't give a reason then don't.
