I am assuming that this is not part of a competition but a case where a theater has heard about your script or you have offered it to them and they agreed to read it.

If you mailed it then after a couple of weeks it would be right to call "just to insure they got it".  It would be logical to ask when you might hear from them.  Beyond that, it is a guess.  It the play reading committee is large, it could take a month or more for people to read it.  If I had not called yet, I would call at the one month point to check how things were going and ask for an estimate as to when you might hear.

One call is not going to bother anyone but I would make sure to get an estimate when you might hear from them again then wait ten days to two weeks past that date before calling a second time.  Each time you talk to them get a date when they might be finished the when you call them back, you can start out with a "you thought you would be finished with your review by now and I thought I would check on how things are going and see if there are any questions."

I have requested unpublished scripts for review.  The problem is that others that I want to read the script may have other things going on and my priority might not be theirs.  I don't mind a call now and then - I understand the anxiety of the playwright waiting to hear but frequent calls worries me that the playwright may be difficult to work with during the inevitable workshop and rework of the script.

Hope this helps.