I disassemble and soak everything (PBW, StarSan) usually for every batch, but sometimes I'll skip one. I never go 3 batches in the same keg without a complete disassembly.
I also disassemble and clean everything after every batch; no exceptions for me. I also use the tube brush for cleaning diptubes, and I even use it on the welded spud on the keg that the post screws onto (while the diptubes are removed). I work under the premise that I don't want to accidentally compromise a batch simply because I didn't clean the serving vessel - this is the reason why bottling was such a PITA for me (OCD/perfectionist tendencies
). I'm the same with my liquid lines, but there have been a couple times when folks were over, a keg kicked, and I had a backup all ready so I just moved the liquid disconnect to the new keg without cleaning the liquid line/faucet/etc. Because cleaning kegs entails a certain amount of prep/cleanup time, I wait until I have 4-8 dirty kegs to clean and do them all at once. I figure it will take me ~45 minutes to clean one keg (beginning to end), and each additional is only like 10 extra minutes per keg so I can get 5 done in about 1.5 hours (which is not too bad for my OCD/perfectionist brain