In the context of the question, I'd say all admins get a break everyday. Nobody is on their forum 24 hours a day.
As for actual breaks from admin'ing.. Thats what co-admin's are for. You can tell them that you are going take a little vacation from foruming and be back in a week or two. Breaks are good so nobody gets burned out.
To answer the question directly, the admin is really always on break. Owning a forum isn't really considered a job and its not like you have a contract with someone, so you are really having a break from your real life job.
You should only take a break if your going on vacation, family problems, internet connection problems, ill/stress and/or if you have another lame excuse. That's when you hire a co-admin(s) that are trustworthy and can keep the forum in good condition.
I like to be on my board daily, several hours daily. That said, there's such a thing as too much and when you start to feel worn out and like a slave, then it is time to bring in more help from your staff. You're of no use to the board, and the board is no use to you, if you burn out.