Setting up UBL for the first time on Marlin 2.x on a Prusa i3, how often should I level the bed using G29?
This is more of a personal preference type question rather than something with a hard and fast rule. 

You should not need to relevel your bed very often unless you have some external force that regularly changes the bed level.

I regularly print objects that take 24+ hours to print, so a 10 minute process prior is not much so it might be worth it to set it up every time if your prints run very long. If you mostly print smaller or quicker to print objects, 10 minutes might matter to you.

I have noticed that my prints fail pretty quickly if I have a leveling issue, and it's not hard to stop the print, clean the plate and relevel at that time.

What should you do?

If 10 minutes doesn't matter that much, go ahead and relevel with every print. If it matters, wait until you start to have issues. See how long it takes for your bed to become unleveled and use that as a guide. E.g. if you can print 10 times before having issues, then try setting it for every 6 or 7 prints.