[bloak]

So the things that might prevent you are:

1. Suppliers have not given you sufficient information for you to be able to prove an upper bound on the time taken. (That must happen a lot.)

2. The system is so complicated that you are not totally confident of the correctness of your proof of the upper bound.

3. The only upper bound that can prove with reasonable confidence is so amazingly bad that you'd be better off with cheaper, simpler hardware.

4. There really isn't a worst case. There might, for example, be a situation equivalent to "roll the dice until you don't get snake eyes". In networking, for example, sometimes after a collision both parties try again after a random delay so the situation is resolved eventually with probability one but there's no actual upper bound. A complex CPU and memory system might have something like that? Perhaps you'd be happy with "the probability of this operation taking more than 2000 clock cycles is less than 10^-13" but perhaps not.

[formerly_proven]

You're probably thinking about bus arbiters in 4.), which are generally fast but have no bounded settling time.