I don't think I can put a number of turns on it. Ultimately it comes down to whether I'm winning my good fights before losing my bad ones, which will vary depending on how outclassed/outclassing I am in each one, and also what the damage is of losing. I care a lot more about Antonian Horsemen crashing through my line than I do about Longbeards.
Yeah absolutely. And my example isn't a straight battlefield representation. For example, if I have Peasant-Peasant-Greatsword, then I am much less worried if that far left unit breaks and is destroyed because the Greatswords have another "peasant shield" defending them. My example was more to illustrate a concept, with a 300 pt unit beside a 100 pt unit fighting two 200 pt units.
I don't think I can put a number of turns on it. Ultimately it comes down to whether I'm winning my good fights before losing my bad ones, which will vary depending on how outclassed/outclassing I am in each one, and also what the damage is of losing. I care a lot more about Antonian Horsemen crashing through my line than I do about Longbeards.
To me tanking usually denotes a unit that can take a beating pretty well, usually high toughness/armor, lots of damage, and/or not likely to run away (Hawk Heavy Infantry). Tanking is usually the act of standing there, getting hit, and not running away. What NE folks call speedbumps or chumps, we out here call tarpit units. These guys are cheap and weak, with their most redeeming feature being they're not likely to run (Zombies strike me as the best example). Oh, and they're cheap.

To me whether a unit is a tarpit or a speedbump (the former connoting that it takes longer to chew through them) is a semantic function of what they're up against. A zombie is a speedbump to Knights but a tarpit to Swordsmen.
Usually it takes a tank (Noun: tough guy) to tank (verb: eat attacks without running). But in my experience, in this game it is better to use a tarpit unit to tank. This can be because of the Pow bonuses everyone gets, because tank units are geared towards grinding wins in a game where you want to win-fast-lose-slow, because the expansion of Green reroll Cge cards makes tarpits less of a gamble, or because the backup rule usually means you can have 2 tarpits for less than the cost of a tank unit, I don't know. I think its probably a confluence of these things.