Tell tale signs bed tape has given up the ghost
Obviously, if the tape is torn or gouged in area that you intend to print on, you will need to replace it to have a good surface. If the damage is well clear of your first layer and will not risk snagging on printer mechanisms during movement, you may disregard it.

I can't really tell you how much bubbling is acceptable. If your prints still come out in with an acceptable quality, then it's OK. If it causes defects that you don't accept, or failed prints due to a loss of adhesion, then it's not.

When I started printing, I used painters tape, and would change it every few prints. To be honest, I was not very careful about protecting the tape when removing a print, and would often damage it. I also often found it easier to just peel the tape from the print bed as a way to remove a print. I don't even think I finished a single roll of tape before I eventually switched to glass/hairspray.

I never used isopropyl or other things to clean it between prints. If you are concerned with cost and waste, I would think this would be even more than simply changing the tape frequently.