Luck in GURPS is based on real-world time constraints, not in-game constraints.
This can potentially lead to the odd effect of giving an incentive to players to "run out the clock" between dangerous events so they can get another shot at a re-roll.
In combat at least, I decided years back to address that.
Luck in Combat
You can use Luck as if each combat takes one real hour, regardless of its actual time.
Luck is once per hour, so that's once per fight.
Extraordinary Luck is once per 30 minutes, so that's twice per fight.
Ridiculous Luck is once per 10 minutes, making it six times per fight.
There is no cool down time between uses - you can use Luck the appropriate number of times, regardless of the time between those rolls. You can use Extraordinary Luck to affect two consecutive rolls, for example.
We do track usage normally, otherwise. If you used your Luck at 2:55 and get into another fight at 3:15, well, sorry, you have to wait - and you might be dead before it's 3:56 and you can take a shot at it.
I like how this makes Luck into a limited resource to be expended in combat, not an incentive to make fights longer so you can use it more often.
Is this fair? Yes. It applies to everyone.
Is it fun? Actually, yes, and it adds a hell of a lot of tension to the decision to use luck. Fights will start and end without using Luck because people keep saving it for when they really need it, and sometimes the fight is resolved before then.
Does this work in actual play? Yes, it does. I've been doing this for so many years I can't remember when we started doing so. It's fine.
I pretty much use this for any "per hour" abilities. We've made exceptions for very long fights (we had at least two epic battles that lasted multiple sessions) and let Luck work a second time in the same fight. But that's rare, and usually not necessary - it's more "generous fate" than "standard ruling."
So if you find people run out the clock in combat to get Luck in, this might be for you. Or if you just like Luck as a resource not a cooldown power, give it a shot.