What percentage of applicants on a wait list for masters program generally get accepted?
The answers to your questions depend on the university/department, but are all reasonable questions to ask. It would be perfectly reasonable to send an email along the lines of


  Thank you for letting me know about the wait listing decision. I understand how competitive the admissions process is. I wanted to let you know that XXX is my top choice and if offered a place I would accept it immediately. As I have also received an offer from YYY that I need to respond to by April 15, would you mind if I checked on my status closer to the deadline?


This will hopefully open a dialog and they might say something like "given your place on the wait list it is unlikely we will know anything prior to the deadline" or "given your place in the wait list the picture should be a lot clearer closer to the deadline.