Good example of why this is going to be hard to balance, heh.
My own experience is that the things actually dont die easily at all, and I often have to remove them myself after the fight is over so I can continue, if I'd put them in a spot that blocked some path or whatever. SilenceBots will just eat the things, but most other enemies dont seem to be able to. The occaisional extremely high health bot being a possible exception, but they're rare. Shieldbots can screw them up as well.... if I havent already captured it for myself. Other than that though, if I've been using them throughout a mission, chances are most of them will still be there at the end of the mission.
4 or 5 turrets in a group is often enough, unless there's a particularly enormous horde coming. They can be placed either in a very long hallway.... where their range hits things before they can get close, or if I'm dealing with something that the simple range just wont kill, I can put a longer string of them down, giving the rear ones more chances to fire as the bots concentrate on the ones at the front. Doing the hallway thing is annoying though. They can also go in a room, with the idea being to catch the enemy bots as they pass through the entrance to the room. The key here is making sure that the bot has to fully enter the room before it can target anything; if it can stand just outside, in the hall, this will not be too effective. When done right, the bot enters, and is shredded immediately. And finally, they can be placed around corners. Generally this one is the easiest to do. Bot comes around the corner, immediately gets shot a whole bunch. With any of these tactics I can also support with my exos if need be.... generally the sniper and brawler are best for this.... but I dont actually need to do this very often at all, it's pretty rare.
My sentries are being placed at trap level 19 right now, I think? I dont always place them in groups, either; sometimes one or two can be used to just add extra damage to my Assault Exo hitting things with his basic laser (which does about 1000 per hit by itself right now) or something like that.
I can show screenshots as well if it helps to illustrate exactly what I'm doing here; I just dont have time to do that right at this moment.