The inactive period for expiration is variable - according to that page:
There's no hard-and-fast rule as to how long a number can stay inactive before it's revoked. There are a number of factors that go into it, and each customer is weighted differently based on usage patterns. We send you a warning message before the number is revoked to give you a chance to keep it active.
I recieved an email about 10 days after activation saying my number was going to expire shortly. Received a second warning today a couple of days later. I haven't been using since I tried it out on day 1. So definitely not 30 days.