How can I customize screen time requests?

When my children send me a request for screen time, there are 4 options: 1) Approve for 15 mins; 2) Approve for 1 hour; 3) Approve all day; and 4) Don't approve.

How can I customize this? For example, if I want to approve for 10 mins, or for half hour, 45 mins, or whatever?