How fast like a injury that would take months to heal would take seconds or what

Think about it, your arm gets broken, a second later its fixed instantly. However with more durability, there will come a point which healing won't help.

With fast healing, it would make the user hard to kill, while slow healing will make death become close faster.

First Choice: Durability level makes you unable to be hurt by nearly all bullets, blades, etc. You can be hurt by extremely powerful stuff like a .50 BMG round, tank rounds, falls from over 10 stories, etc. Your healing is slowed to the point a simple scrape or cut would take over a month to heal, moderate injuries months, and severely broken bones almost a year. You will age normally.

Second Choice: Durability level makes it so even a dull knife can actually cut you, a simple slap will leave a bruise like you were hit by punch, falling down stairs would break bones. Your healing would be fast enough to heal minor cuts and scrapes within a few seconds, moderate injuries in a few minutes, and severely broken bones within a few hours (the bones would reset themselves). You would age normally