Cache timing

[Tashina]

Hi there, I have a question about cache timing.

If I have a complicated derived table that takes ~20 minutes to run, and I want my cache to be good up to the latest hour, should I have the cache bust and refresh 20 minutes before the hour, on the hour, or after the hour? 

Basically, I’d like to know how to time a cache clear and refresh to make sure the new cache is good on the hour, for a full hour.  Hopefully my wording makes sense.