Here's some information from Twilio's Twitter (dates back to March 27, 2014): https://twitter.com/twilio/status/449231807888621568
"There is no time limit, you can go at your own pace (unless you outpace $30) . More free trial info: http://bit.ly/1fo64aN"
You can check your current balance here: https://www.twilio.com/user/account/usage/primary