How long the time be between end of plan & 1st race

How much recovery time should I schedule between the end of Intervals for XC Mountain Biking and my first race?

You could have your first race the week after you finish the interval plan. So that would be the 7th week. Just make it a similar week to the regeneration week.

Then you could start the XC mountain bike in season plan!

This plan has race dates built into with proper off days, interval work and openers leading into race weekends. More if you have several weekends of racing in a row.