The trial period is 30 days.
The proper way to continue using it after 30 days is to register it.
At one point you could continue to use mIRC unregistered after the 30 days (as obviously pointed out by some of the earlier screenshots), but it appears that Khaled has gotten tired of people using it past the 30 day mark and has implemented a mandatory "pay or don't use" type policy after the 30 days.
Note that only Khaled can confirm or deny this outright.
