Part of what makes the Legacy Challenge fun and leisurely is that it's very open ended. The beauty is that the choices you make with your family are going to be different than the choices anyone else makes, and after a while the family sort of takes on a life of it's own.
I have played several variations of the Legacy Challenge, often the difference will be that 1 trait is required for all family members (except for spouses), that you have to control only 1 sim per generation, that you are limited on which items you can buy or how you build, or randomly roll for traits. And then of course there's how you score it. Do you give points for art work of each family member, do you subtract points if certain events occur, do you give points for wealth, or LTH points?
The framework in all of these are pretty much the same, and it seems to be the part that makes people want to try it. That you have a founder, who marries, who has an heir, perhaps siblings, and then the heir marries, has kids, etc. Leaving LTW's up to the player or roll of the dice.
Anyway, just thought I'd share the thoughts crashing around in my brain as I read this.

Personally, I think a set time frame would be a mistake, it would rush the participants and take away part of what makes a Legacy fun.