How long should I give high school actors to memorize each scene of their play?

My director says once we block a scene, you have to be off book for that scene at the next rehearsal

give them 10 minutes

I just wrote/produced/directed a play for Fringe.  Most of my actors were in middle school.  The three leads each had 140+ lines.  It was a 42 page play.  We rehearsed one day, 45 mins a day for 7 weeks and were off-book on week 4.

How difficult is the dialogue, and how much blocking do you have? Are you taking a break for Christmas?

I would suggest that giving the cast and understudies until after Christmas to be off book gives you pleanty of time. I also earnestly advise you to run through on the set well before the day. I was doing Shaw's "Passion, Poison and Petrifaction" as the Copper, and the day before opening we discovered the door I was supposed to enter through was painted on the wall!

And if they don't know them in the time you allotted, then what? You are a student director this year, so I'll assume you have been on the other side. Have been around high school level theater and such. Have seen the varied commitment levels, the kids that are over scheduled, that try to do to many things at once, the ones that are there for the social aspect instead of the art or love. You have seen those who struggled and those that instantly picked it up.  Basically, you are stuck with the people you have and they will all come along, at their own individual rates. You can certainly set an expectation of when you would like them to know their lines, but a hard line time. Is only going to cause you grief. So you set the rehearsal schedules up, you communicate, in advance and often, what you hope to cover, and you pray. The do their part.