You want to take it regularly especially during year 1/2. After that maintenance comes into play. Occasional missed doses don't have any negative impact, takes a week for DHT levels to return to normal after discontinuation. May even take 2 weeks for people who were on it even longer. For dutasteride users, it can take months for DHT to get back to normal.
As for "catch up hair loss," reading up on this a while go it seems it starts like a month after coming off minoxidil but coming off finasteride it takes much longer to hit you for some reason. Like 3 months to over a year.
I would personally get twitchy 2 weeks without finasteride though.