When it gets to looking too grungy, or starts ripping that's the time I retape my bars.
To retape it right? go slow, be careful around the brake levers some of the cork tapes
can tear/break if pulled too tight.
I always fold the brake hoods back so when I'm finished the hoods partially cover the
tape, looks very good.
Personally, I like cloth tape, shellacked for that classic look.
Marty