There have typically been 2-5 updates for each perpetually licensed PD version over the active life of the product, which is on the order of 2-3 years. CL has recently started issuing monthly updates for the subscription version (PD365) but so far they haven't released any patches for PD20.

You should be notified when an update becomes available when you open up PD, and you can also check the forum and the software updates page for new patches.