As Artashes said - whatever you do, don't say FREE for 10 days. This will invite every spammer and hacker into your network and leave you working doubletime to keep things ok on your system.
A 30 day money back seems pretty standard these days with hosting companies, but I've found others offering 7 days, and some with 500% money back guarantees for 45 days. It all depends on your marketing strategy and what you're trying to accomplish.
For me, only software is good for the try before you buy type system - beyond that, money back guarantee usually works on a service level.