I would have put how long can you survive but it would have ruined it for people...
I played it on easy and lasted like an hour and a half before I got bored. It's much tougher on normal because ammo runs out quickly and so many Elite hero's and wraiths and phantoms shoot that it's hard to pick up their weapons.
On heroic, utilizing the boxes for cover in the room, I lasted about 26 minutes before zealots and hero (white) elites came in numbers.
Legendary just seems ridiculous and I usually die by the first wave of elites coming in.
How about you guys? On normal I would have survived a bit longer, but I tried to destroy the wraith shooting at me and preventing me from picking up ammo and it said "return to battlefield" and I died. I like that Bungee made it so the more of them you kill, the more noticed you become and the harder troops come out.
I let myself die as quick as possible. I don't see the point of it. You're gonna die the same way anyway regardless of how many dudes you kill. Unless you get something special for lasting for awhile, I don't think I'll bother. I like to just get it over with as quick as possible. I don't think the game will care, hopefully.
Serious... I've seen someone last well over an hour on heroic utilizing the boxes method. I got like 20+ minutes You have to be clinical with your shots. And run and quickly grab whatever weapon they drop (preferably plasma pistol to take away hero shields) and hide behind the boxes again. If you go out, you are pretty much a goner because by then at least 2-3 wraiths are near you and phantoms and elite heros will shoot. On legendary, the first elite minor wave usually gets me as well.