My DD1 is allowed to play outside alone whenever (downstairs, fenced yard, very limited vision from inside) and does at least once a day. She is around 4 and a half but started when she could reliably get back inside for toileting so late 3ish. Usually just plays in the sandpit or chalks our driveway. Never had an issue. I'm only comfortable leaving DD2 (18mo) downstairs very briefly, say to use the toilet or grab something upstairs with her sister ready to alert me to troubles. She's a climber and it could get bad quick due to her age.

My girls play together upstairs with limited to no supervision while I do dishes or other assorted jobs in different rooms. The only rooms they can get to, they can use (toy room, clean art room, book room) so they can't get up to much mischief. DD2 is the rougher one but DD1 has restraint and being faster can escape easily so they never have any real dramas.

Definitely think it is good to give kids some freedom and independence when appropriate to develop responsibility and self regulation, plus problem solving and conflict resolution between siblings. As well as unstructured play for working that imagination and exploring their personal creativity!

As for an amount of time, I'd say for DD1 about 30-50% of her time is unsupervised depending on the day. DD2 around 15-30%.