How does TubeBuddy calculate the average of the last 5 videos? Over what time frame are these values calculated? How often does the keyword explorer update its values? 


It's the median views of the last 5 videos that are at least 3 days old.  It's updated every 2 weeks.

YouTube's analytics data has a 2-3 day lag before it's available, which is why there is a 3 day threshold.