Per short rest. I reckon it sees use as often as Mage Slayer. Multiclassing to Battlemaster is better though.
I definately get to bop a baddie for casting next to me more than 1/short rest. Probably average of 1/encounter, when the adventure has a lot of spellcasters. Still a couple times per rest with light spellcasters, bc the caster can’t get away from me most of the time, and they have to either cast and pay or not cast and let their team get wrecked. Being a Gnome Swashbuckler with Mage Slayer really is a great caster-Bane build.
But, YMMV. We tend to add casting to critters, as well, and even a melee guy with melee cantrips gets an OA before they realize I can do that. And they can only use escape spells so many times. And if they use shield then that’s just as good for my team, since they are burning even more of their spells ineffectually.
The PHB says that sneak attack only works once per turn, so I'm not sure what you mean. Unless you are saying that the swashbuckler is using sneak attack on opportunity attacks as well.
Yep. Rogues get Sneak Attack 1 per turn, not 1 per round. So, any Attack you make on another creature’s turn qualifies as long as it meets the normal SA requirements. Since Swashbucklers are very good at being exactly where they want to be, it’s usually easy to be within 5 feet of a caster when they take their turn. So, unless they have misty step, they either have to move and cast, cast without moving, or disengage and move. Two of those get you an opportunity Attack, the other burns their action on the disengage.
I like to also pick up Sentinel with this build, providing a whole other line of opportunity Attack...opportunities.
If they cast, move away w/o disengage or teleport, or attack an ally within 5 ft of me, I get that sweet opportunity Attack.
Now if only I could guarantee advantage on those attacks!