What kind of gameplay would be added by such a feature? For example, you launch a rocket but it explodes, you go to the VAB and design a new one. But you can't launch it, you gotta stare at the screen blankly for X minutes until you can. (Or design other vehicles in VAB/SPA, or control a craft already in orbit if you have any) Absolutely no point in such addon unless you're extremely hardcore and willing to put such (realistic) restrictions which have no contribution to gameplay. On the other hand, a system for career where launches cost resources, and those resources might take time to recharge/re-obtain, might be more appropriate, as long as you can still timewarp until you get the required amount of resources.(Perhaps there could be different kinds of resources and a limit of max resources), but an arbitrary time limit is no good.
