Don’t cover an entire bike with it, just focus on areas that are vulnerable to damage from abrasion. For example, apply to sections where cables rub or where you might get heel rub on swingarms etc.
Some folk do the underside of the downtube, which is another area you might want to look at if you’re worried about gravel being thrown from the front wheel and chipping the frame.
It is easy to apply if you take your time, clean the bike, use a sharp knife to cut it (and I mean surgical scalpel sharp, not Stanley craft knife sharp), heat it up to get it nice and pliable, and make paper templates first to ensure correct and tidy coverage. Don’t expect it to be a quick job, and don’t do it in the dusty garage/shed, or with mucky hands.
As for amounts required, it’s impossible to say. Depends on how much you decide to use on each bike.
