In over 3 1/2 years I had never cleaned out my sump before. I recently shutdown my 60g, and had to clean everything out. I was surprised to find that the sump contained about 2 inches of muck! The tank ran like a dream though and the water was perfect.
In the future, I will probably keep a closer eye on it, but if there are no issues, I don't see a reason to clean it.

PS - Lots of bristle worms ect, living in that muck, so one more reason to leave it be.

I have never cleaned my sump until the move and don't intend to as my sump is always filled to the top with rubles and live rock that doesn't fit proper in my display.
Next build I will have 2 sumps for the same reason filled with rubble rock as a sess pool for pods feeding my main display.although my fragmentation tanks act as sumps to and keep those as clean as possible.

I do my wc through my sump.

Filthy, but seems to work for me.

I had a big one too, except he found a way into my reeflo pump one day...:twised:

Never cleaned it and no plans to do otherwise either

The largest chamber of my sump is drained for water changes, so I siphon out any gunk that's in there. I pump fresh RO water directly in to that chamber and mix the salt in it so it pretty much toasts any critter that was unfortunate enough to have moved in during the time between water changes.

The other 4 chambers have never been cleaned and probably won't be because they're covered with sponges, fan worms, small mysis shrimps, pods, etc. The chamber my skimmer is in has collected a dusting of detritus at the bottom (that has precipitously shrank since i started dosing MB7) that is literally alive with bristle worms and limpets.