However much my preschooler wants.
My two year old insists on “doing spelling” everyday. He gets the whiteboard and dry erase markers just like big brother, then we work on his colors while he uses each one to scribble.
I have plenty of activities for anyone who wants them, but if they are away playing “magic fairy kingdom,” I am even happier.
My 4 and 2 year old have about 2 hours of school per day. I’ll have them work on ABC Mouse website, and after they have printouts to work on which is usually coloring, matching…Also a good book to read to them before nap time and bed time 
Every now and then I’ll pull out Pocket Flash Cards with the numbers 1 thru 100 and another deck with Shapes and Colors. There are a bunch more activities but these are the basics.
I am currently using COAH letter of the week curriculum with my preschooler. We do M-F for 1-2 hours. It’s never gone more than two even when she asks for more school lol. I just feel any more than two might be pushing her so I find some other fun activities or take her to the park. We live right next to one and these two women have an in home day care and bring their kids to the park near me. So my little one is able to get about an hour and a half to play with about 10 kids her age! 
Right now it works perfect and really helps with socialization and learning to share her toys with other kids her age!
I feel like I started very young with my daughter, because she was so interested in “schooly” things. This year I set up a preschool curriculum since I knew it was important to her and I didn’t want it to fall by the wayside with a newborn halfway through the year. Then she started really wanting to read so I got all about reading 1.
All that together I keep it very flexible and I would guess we average out 1 hour 3-4 days a week.
Plus lots of reading together-that’s not school for us 
I have never formally taught my kids preschool or paid for them to attend preschool. That is until son #4. Although this school year is still very laid back, I have started doing more targeting teaching. Mainly because they believe he has Developmental Coordination Disorder & he has trouble with cutting, coloring & writing. So we’re mainly working on those elements to build up the muscles in his hands for school later on. All the other stuff ABC’s, math & such is what I would normally do with my kids. I never realized what I did was homeschooling them. I thought everyone did these things with their kids to prepare them. Wow, I’m long-winded. Basically, Jack’s school has no set time right now. I watch & see how he’s acting. Then gauge from there what I think he might be up for. Next year when I start homeschooling his older brother (1st grade), I’ll put a more formal school time on our well, school time.
My preschooler does 40 minutes of her online preschool and 30 minutes reading with me. We do this Monday thru Friday. So we are done in any an hour. Sometimes she likes to be included in her sisters lessons but not always.