If something needs to be done, life stops until it gets done, whether it's schoolwork or something else.
I want you to get to play outside my dear, but your assignment needs to be done first. You can do it in the 15 minutes it really takes and then go play, or you can stretch it out to 4 hours and miss playing. Your choice.
Every once in a while, my oldest will test the waters and cry and complain. And when he sees that's not getting him out of it, he sucks it up, buckles down, and whips through the work, usually finding that it's not nearly as much of an imposition as he was trying to tell me it is.
I feel the same as laundrycrisis--I don't feel bad at all about being strict, because my kid's life is tremendously free and flexible, they get *tons* of free play, and if they choose to their schoolwork can be done in a very small amount of time. I do have sympathy for my oldest--third grade seems to be the year academics get serious and he's in online classes that take 3 hours (with breaks in between). Still, at maximum, if he's doing his work without messing around, he has a 4 hour school day, with plenty of time in between the work.