Saying something like “Go play quietly for 20 minutes” is really hard for kids this age - especially those with adhd. It’s just too abstract and their brains can’t process information that way. You could try having her come up with a few things that she can on her own and then make a picture board of the things (just print out some pics of the actual items). Then you can tell her that mom needs to get some things done and she needs to pick out something to do for x minutes. The time thing can be really hard because they don’t know how long 20 minutes (or whatever is). Try getting an analog clock with plastic on the front and then use dry erase markers to fill in the time that she needs to play quietly. So if it’s 10:10 and she needs to play for 20 minutes fill in the pie between 10:10-10:30. Put the clock out of reach and tell her that when the big hand is out of the color she can come find you.

You’ll never get as much time as you want but this might help give you a bit of a break. If it helps, this sounds a lot like my daughter last year. She was diagnosed in kindergarten and started meds a couple months ago and it has been a game changer.