Very variable. It seems to depend on time of day (and time of week).
Sometimes you will get a dozen snipes in an hour, other times you might only get 1-2.
I see no reason to boost barracks though. As a TH9, I usually use m AQ to snipe, and only drop troops if it is trapped Which is infrequently enough that they will replenish without boosting.
As for loot - in M3 every snipe is a "free" 100K lix and 500DE. The gold will of course depend on how many "nexts" you have done, but you will still make some gain unless you have done 130 (at TH9).

