Number in time periods

You can use either, though less has the advantage.

There's a bit of a long-running argument on the use of less for countable items, but both sides agree that it can be used with real numbers, such as "8.3452 minutes" or "seven and a half minutes" or even "5 minutes" in a context where it's just by chance that our measurement comes up with a whole number of minutes.

That's assuming that you don't mean a whole-number of minutes only (i.e. it could be 3, or 5 or 9 minutes, but it couldn't be 5.94 minutes). Then you're back in the less / fewer debate, and probably better off siding with fewer. It would be an unusual case though, and probably need a less concise explanation.

When it is a countable case "Ten items or less" vs. "Ten items or fewer", it gets more controversial. Some maintain that when dealing with whole numbers, we can only use fewer because that does relate specifically to whole numbers. There's a long-standing use of less with countable cases though (at least as far back as about 888CE).

Because "less" has been used this way for 1000 years. In 1770, however, a certain Robert Baker wrote about his preference--that he uses "fewer" for number among things that are counted and "less" for quantity or amount among things that are measured. To cut the long story short, this became a rule, which, unfortunately, seems to have a million exceptions.

I urge you to watch this video from Merriam Webster. http://www.merriam-webster.com/video/0013-fewer.htm?&t=1391533251