How long can you keep a POI waiting?

[cobalt27]

To start, I applied to 8 M.S. programs, all fully funded. School A and B invited me to recruitment weekends for this month, and I have booked flights to attend both.

A POI from School C recently emailed me, with what sounds like an unofficial offer. She explained that my app was well received, described the financial aid package, and asked how early I would be able to commit to attending. She explained that if she goes through the process of extending a formal offer to me and I don't accept it, the school will make her wait a few months before she is able to offer the spot to a different applicant. 

What is the etiquette in this situation? I still intend to participate in the recruitment weekends at Schools A and B. Is it advisable to tell the POI at School C that I need until at least March to formally respond? Do I offer her any details as to why, or is it implied that I am considering offers from other programs?

Any advice from people who have been in similar situations is appreciated! All things considered this is a great "problem" to have and I want to be sure I handle it as smoothly as possible.