data structures – How do we pick and switch hash functions from a universal family?

When we have a universal family of hash functions, it gives us a few useful mathematical guarantees. But, if we pick a specific function from the family and use it all the time it’s effectively like the family doesn’t exists and we only have this one function.

Therefore, the only logical thing seems to be switching a hash function every once in a while (picking a new function from our family). If indeed this is what we should do, how often should we switch functions? How should this time period be selected and is there an optimal time period for different tasks?