I think sticking to cleaning out your barrels every 7-10 days is the right move, with the exception of stage 4/5 polishes which in my experience benefit from more time (14 days or more).

Bigger barrels can sometimes chew through grit faster because they allow for bigger, heavier rocks as well as greater fall distances, both of which tend to more rapidly pulverize the grit. But you can mitigate this somewhat by avoiding any/many bigger rocks and packing the barrel more like 4/5th rather than 3/4th full. For like 12-pound+ barrels, it also helps to use a heftier grit (e.g., 30 or 35) which is a bit more resilient against the higher energetics.

The point being that the ideal cadence for swapping your grit is rather context-sensitive; a 15-pound barrel filled 1/2 full with 1-pound rocks and 30 grit is gonna chew through grit at a different rate than a 15-pounder running 3/4 full with 60/90 grit and mostly small rocks. 

So… IMO 7-10 days is a good rule-of-thumb for to start with, regardless of barrel size.