Good question. Not much of an issue for me, since I’m doing mixed fermentation. And with adding fermented beer to the barrel, it builds up pretty slowly. I’ll probably rack everything out this spring and give it a rinse with boiling water, and then re-fill. But the only issue with the brett in there is how much volume the trub layer takes. This should be three years of trub, but it doesn’t seem to be too bad yet.
If you’re going to keep your barrel clean, then I’d probably want to dump the trub once a year or so. Rack all the beer out, add some boiling water, and shake/dump once or twice. For a 5 gallon barrel, it should work about as well as steam cleaning.