It appears that fixpacks are now planned to be about once every 6 months, but there is not a definite schedule. Previously the were released about once every 3-4 months. For emergency fixes (security problems, and certain other critical fixes) they will issue an "a" or "b" fixpack such as Fixpack 4a in-between the major fixpack releases.
The install depends on what version you are coming from, and whether there are any changes to the table and index page layouts that need to be applied (this is usually only when upgrading from a previous release, like V8 to V9). If the pages need to be reformated, then the time it takes depends on the size of your database. Otherwise (V9.5 FP3 to V9.5 FP4a) takes considerably less than an hour.
M. A. Feldman
IBM Certified DBA on DB2 for Linux, UNIX, and Windows
IBM Certified DBA on DB2 for z/OS and OS/390