Team should keep its challenge if it succeeds, no matter whether it's their first or fifth. Why penalize them for the number of mistakes the refs make throughout the game?
So if you win your challenge, you save a charged timeout, do you also keep that challenge or is it spent regardless ?
If you win your challenge, you get a second one. The second one is gone when you spend it, successful or not.
I contend that you should keep your challenge as long as you keep winning them, no matter how many that is. As it is, if the refs blow a crucial call in the fourth quarter, we're hosed through no fault of our own because we've already had to correct two of their screwups tonight.