Request of me twenty, receive of me twenty.

Request of me sixty, your behest will be fulfilled.

Request of me eighty, you'll be given the same;

but ask for one hundred, and you will get only sixty.

What am I?

Are you

minutes?

Because

20 minutes is twenty minutes, 60 minutes is sixty minutes, 80 minutes is eighty minutes (or 1:20 hours/minutes), but 1:00 is one hour or only sixty minutes.

More specifically,

numbers entered into a microwave (that means seconds/minutes instead of minutes/hours, but the idea is the same).

• Precisely! Well, nearly - the intended response was "microwave", but provided the correlation existing between the two, both answers are entirely valid. Nov 20 '19 at 8:07
• I never used a microwave, so I wouldn't know that part :-) But 100 -> 60 seemed like too much of a coincidence to not be connected to this answer. Nov 20 '19 at 8:09
• Do most microwaves actually allow you to "request eighty"? Nov 20 '19 at 9:50
• @GarethMcCaughan Yes. In seconds, anyway. And with the specified behavior, all of which applies exactly as described. I expected that to be the answer in fact (and OP’s comment seems to confirm that I was on target).
– Rubio
Nov 20 '19 at 13:35
• Interesting. I'm pretty sure mine doesn't. Nov 20 '19 at 13:35