There are a number of triggers to convert a user from a free trial to a paying member ranging from time based (15 day trial) to performance (number of applicants). Here's a quick read that reviews most of the mechanics.
http://unbounce.com/email-marketing/convert-free-trial-users-email-marketing/
CrazyEgg has a longer, more thorough post that discusses how to convert free trial customers to paying customers.
https://www.crazyegg.com/blog/convert-free-trial-users-customers/