In TMW Legacy, the battle have unlimited time, but a limited number of waves. Difficulty is also mostly constant.
I have to admit this statement isn't fully correct: each round can be 2 minutes max, level limited to about 2100, therefore finite number of rounds takes finite amount of time and therefore "limited time", though it "win by running monsters to death" (a rather funny feat, eventually achieved by few ppl). You don't lose health - if you keep moving (it's anti-cheat feature: some smartasses got idea it's cool to sit in arend and do nothing useful for gameplay, getting boss points anyway - which is quite wrong in regard of encouraging MMO gameplay).
I hope this thing also supposed to have "winnable" modes? Because, you see, losing battles is, generally very discouraging in terms of MMO interaction: once battle lost and everyone/most ppl dead, "they had it enough" and MMO action ceases. Which isn't good in terms of gameplay. So I'd say "primary" modes have to be "winnable" and at least somewhat "rewarding". But as some extra to occasionally try this "last man standing" is definitely nice idea. Still, I hope things wouldn't be set in stone, so if other/even better ideas would come - why not?
P.S. if someone crazy enough, lol, damn, how about CTF (PvP-enabled) mode/arena? (well, I can imagine coding something like this in MMORPG is very challenging though, and I never seen RPGs doing that, so I'm not sure it's viable idea - and yet it can be fun).
N.B. CTF stands for Capture The Flag, a rather funny team play mode often seen in first-person shooters. I don't see what fundamentally prevents it in MMORPGs in 3rd-person mode (not sure if someone implemented it ever though). Technically it like a team vs team "PvP" with extra goal of stealing enemy's flag (normally they are of 2 different colors, matching teams, but it's an abstraction) while guarding own base to prevent enemies team from stealing your flag. Person who "steals" flag is "flag carrier" and obviously becomes #1 target for all enemy team at all costs (so it grants some adrenaline). If flag carrier shot, flag returns to where it's belongs, it's base. If flag carrier makes it back with enemy flag to their own base while own flag stays there, team wins round. At this point state is reset and new round begins. Killed players eventually respawn and rejoin to maintain overall heat of battle, but obviously getting killed interferes with goals a lot, so if enemy team manages to arrange kills in a strategic fashion they achieve objective with far less impediments.
On game mechanics layer I can imagine whole thing can even be tracked by server scripts only - though it likely quite complicated and I'm not sure how fast it can respond.
Seems some even play it in real life, outdoor - https://en.wikipedia.org/wiki/Capture_the_flag
What's the point? In this thing it's not enough to just kill all enemies you meet. It rather takes some tactical and strategic thinking and team play to win - that's what makes it a bit more funny than ruthless carnage alone. Usually it implemented the way it possible to lose even despite being good at killing enemies if team gives no crap to mentioned goals and killing right ppl at right places at right time. Some few random deadly monsters doing random kills can probably make thing a bit more fun as well.
p.s. on 2nd thought, I guess I shouldn't vote for...Unlimited time since totally unlimited time allows player(s) to "usurp" quest exclusively. I'll think there should be global timeout in any case to prevent "DoS attack" approach. Yet, somehow voting doesn't lets me to change mind on that...