Typically we tell our athletes anything over 5-7 days and it’s in your best interest to start the program over. That would be our suggestion to max your gains. However, if you have no issues getting through a workout without fatigue or soreness, anything less than 5-7 days, and you should be fine to pick up where you left off, but just keep in mind that it may impact your results.
For example, you miss a workout but were still doing something active on that day, you can just pick up where you left off and skip the workout (the activity accounts for the missed session). However, if you were sick, or injured, you'd want to wait until you're fully recovered, then either continue the program where you left off, or start over. Keep in mind what we've mentioned above, as the program is designed to work best when done without interruption!