my 6 yr old can just about do around 1.5 hrs of independent play/pottering on a good day. He's an EWer and is up everyday around 6am and usually on the weekends does not come to us before 7.30am. This took a LOT of support, encouragement etc over the years, we started around age 3.
By independent play I also mean we have self-care in that 1.5 hrs - toileting, dressing, getting brekkie for himself, colouring, drawing, legos, puzzles etc. Max he can do 2 hrs.
My 3 yr old is rarely "alone" as she always has or mainly has him around. If left to her own devices I think she could do around 40 ish minutes on a very good day but she doesn't get much opportunity.
S x