I like a high tip, so usually when its down to around a nickles thickness.
Depends on my mood really, I change my own tips and if I feel like putting one on I will. Been playing with tailsman tips, so change them out about once a year or so.
I tend to scuff my tip more than some would recommend, but I like a consistent shape and since I play and break with the same cue it flattens out a bit over time.