Microsoft Office does this too. I think you get 100 runs, which is almost too long, since people may use the app for 6 months to a year before hitting the wall. Or maybe that is the point.
One advantage of a really long trial (say, six months) is that if your product/service has become useful to them at that point, it's unlikely they'll bail out. As the deadline approaches you can send them an email or on-site alert reminding them that they'll lose the time/data/learning/connections they've invested in. That kind of pitch doesn't hold water for a 14-day trial.