Oh... I usually break them down completely every two or three beers, but that's just me.
When you get into the swing of things, it gets easier. For me, breaking down and filling up the kegs goes something like this:
1) Keg floats (empty)
2) Open keg up, spray remaining gunk with garden hose & sprayer
3) put lid on, shake
4) dump
5) if gunk remains, reach down into the keg with a scrubber sponge, break up gunk and repeat (2)-(5)
6) If needed, scrub down the in-sides
7) Final rinse & dump
8) remove posts, poppets, dip tubes, lid, and o-rings. Inspect, scrub if necesary, place in sanitizer bucket
9) siphon sanitizer from bucket to keg (bonus: your siphon is now sanitized to put brew in keg)
10) install posts, poppets, dip tubes, lid, o-rings
11) use CO2 and picnic tap to push sanitizer to bucket (bonus: keg is filled with CO2, n ooxidation worries)
12) siphon new beer to keg, close keg, put on CO2 to carb
All in all, probably takes me 30 minutes at most.