It depends on the type of license you are talking about and when they were purchased - Cisco has changed how they handle software licensing for their telephony products.
The DLU is the older licensing model - where they licensed each device. Device Licenses never expire, but if you do not continue to pay the annual SmartNet charge for maintenance and upgrades, then you cannot get TAC help or fixes for any software bugs.
Cisco now sells the CUCM telephony with a user-centric software licensing model instead of device licensing - although in some cases you can still get proposals for both. As above, the user license itself does not expire, but SmartNet (annual fee) must be maintained to get support.
The core software services do have separate licenses since version 6.0, but most installations I have seen do not have expirations (as in software stops working) but patches and updates require the SASU (software application support plus upgrades) fees - so effectively you have an annual license "renewal" if you want support.
A good overview of Cisco Licensing is here:
http://searchnetworkingchannel.techtarget.com/generic/0,295582,sid100_gci1316825_mem1,00.html(free registration required).
