I think practice is key. Small pieces first (like, "all your toys on the floor need to be picked up by dinner time," or, "please read two chapters today") and then bigger pieces as he gets better at judging his own pace.
And with DS, I've given him responsibility for specific types of things (like his book group reading) while helping to schedule others for him, just so it doesn't get completely overwhelming. That way he can work on keeping up with bigger and more nebulous assignments, but not have to keep up with many of them. As of last year he was almost-independently scheduling his science fair projects... multi-part and enormous, but also really well-defined.
I do think this is developmental to a great extent, but once they're developmentally capable of it I think it also requires a lot of practice and sometimes direct instruction.
