When I was in Germany a few years back, I visited some serious fish stores there and asked about this subject. The 6 or 7 people that I spoke with, all had DSB's setup for 10 years or more. I was told in order to keep things clean, some would siphon out 25% once a year and replace it after a good cleaning, others told me that they would simply stir it up and run a canister filter for a few days, and the rest told me to go with an inch or two and to use lots of critters like snails and crabs.
My DSB is about 4 years old now, as I replaced my entire SB after two years into reefing-don't ask
I plan to keep my 2" DSB until I see reason to change. My only concern is that if we eliminate or reduce our DSB's aren't we not going to see these harmful elements transfer to our LR or worse to corals and fish
To make a final comment on this great thread, I am planning next year to clean 50% of my entire DSB. Sure this is going to be a major rip-out even in a 27gl, however I would rather spend an entire day cleaning my reef rather than loosing 6 years worth of hard work and beauty.
cheers, Rich