I try to do 30 minutes a day. It doesn't happen every day for each child but I try really, really hard.
Part of the reason for this is because of reading I've done (Playful Parenting, The Challenging Child) suggests that this is a really good way to connect with your child and keep that connection strong. 30 minutes a day of "floor time" keeps the connection going. I WOH and so it's important to me to stay connected.
And I will confess that I don't mind pretend play. I love to see how my kids' minds work. Since ds is obsessed with playing with school, it gives me insights into what's going on at school. What happens when kids act out? I act out and find out. What happens when your work is illegible? Write illegibly, and ds writes a note telling me to "rerite this, it's too sloppy!" For dd, it helps me see what issues she's working on too -- family themes, relationships, what it means to be a caretaker.
OK, playing "Tina" (the child at school for ds or the child my dd is taking care of) every day for weeks on end does get old. (And has led me to begin to loathe the name Tina -- apologies to all Tinas out there!)
A side benefit is that they have more patience when I am truly busy. A week ago, I had exams to grade, we had friends coming over to dinner and the house was a disaster. I did not have time to play on Saturday. I explained this to them, and they did not pester me to play. Their cups of attention were full enough to weather a day or two where I didn't play with them.
Now, if I were a SAHM, I would try to do 30 minutes a day, but I'd be comfortable leaving it at that, even though I might theoretically able to do more.