Stimpacks have a diminishing returns. (Beta 144 and 117) - this effectiveness returns "over the course of the day" -

Should this be 1 point per 4h48m or all 5 at once, or conviently round at 1 per 4 hours?

If all 5 at once, should that be after sleep, 24 hours after first pack, 24 hours after last pack?

Yeah, I know, nitpicking. My groups only just decided stimpacks are a good idea.

I would say once you've had the opportunity to rest, Stimpacks regain their usefulness. Otherwise, you get into completely uneccessary (and anti-climactic and tedious and uncinematic) record-keeping.

I would recommend you give this guideline, but lead with the caveat: "Guys, I'm the GM, so I reserve the right to call an audible about Stimpacks. Sometimes the scene calls for a higher stress level and your systems might not have recovered from the stress of the previous 18 hours just yet, so your Stimpacks won't work as well until you've had some real downtime. But, for a general guideline, when you go to sleep for the night and then wake up the following morning, Stimpacks will be fully effective on you again.

Obviously you should run it however you want! But whatever you do, I would totally give your players the caveat. Let them know you're not trying to screw them over; you're just trying to make the game fun and exciting and sometimes that means making it more challenging.