CPUs are released every quarter - there is no official release schedule for PSUs. Once you apply a PSU, you cannot apply CPUs - you need to choose which "path" you want to take - the CPU path or the PSU path.
See MOS Doc 854428.1 - Intro to Patch Set Updates (PSU) (especially Sections 3 and 9)
Also see this related thread - Re: PSU Vs CPU
But, according to MOSC doc id 854428.1
CPUs are released every quarter - there is no official release schedule for PSUs...
...PSUs are on the same quarterly schedule as the Critical Patch Updates (CPU), ...