You can see the expiry date and time via 'show system licenses' - example below from a SRX which previously had a evaluation license installed.
IPS will stop function after the end date and time in the license.
Subscription licenses for the SRX series can be bought for 1, 3 or 5 year period up front. You will get a bit lower price per year if you buy 3 or 5 years (between 10% and 15%). Subscriptions are named SRX1500-IPS-1, SRX1500-IPS-3, SRX1500-IPS-5.
user@fw> show system license
Licenses Licenses Licenses Expiry
Feature name used installed needed
dynamic-vpn 0 2 0 permanent
logical-system 1 1 0 permanent
remote-access-ipsec-vpn-client 0 2 0 permanent
License identifier: JUNOS845638
License version: 4
Valid for device: CV1316AF0108
idp-sig - IDP Signature
date-based, 2016-10-17 02:00:00 CEST - 2016-12-16 01:00:00 CET