Each Raid Orb will replenish or regenerate for exactly one (1) hour. That is a total of three (3) hours if you have used all of your orbs.
To be able to maximize your Raid runs, we advise that you do take note of the time you completely use your orbs so you can re-do it again the moment it fills up.
If you can not wait for the Raid Orbs to replenish on their own, there is always an option to use Gems.
If you want to know how to purchase Gems, you can refer to this article.