I've been thinking about making a total conversion mod.
It would be a mission with a lot of content, probably one to two hours in length.

Essentially, you'd control a single character, who, over the course of the mission, could buy better weapons and upgrade his armour.
If you die, you'll respawn, but with the worst armour, and basic weaponry, if any. You will, however, be able to recover weapons from your corpse.

As I've said, it will take quite a long time to play, most likely. I could be grossly over-estimating the length, but I intend to put a lot of content in.

Now I was happy with all this, until I thought about saving. I can't think of a satisfactory system for saving, so I figured that it could be a mission designed for playing all in one sitting.
I'll recommend playing it with a clean copy of CC to hopefully eliminate crashing, but it's possible that that'll be an issue for some of you anyway.

So, I have two questions.
1. How long would you be willing to set aside to play a mission?
2. How long, in your experience, does a Vanilla or near-Vanilla copy of Cortex Command run for without crashing?

Only question 1 is vital, but if you can reliably answer question 2, that is very much appreciated.

Well, saving is perfectly possible, but if you absolutely want to restrict to one 'mission' I'd say keep to within 30 minutes, roughly.

I've never had CC crash due to memory running out, but I also don't usually play CC for extended periods. Inside of an hour is definitely doable on any of the computers I've ever played on, though.

I'd have to agree with Grif on this one. Also, you may consider implementing a checkpoint system as an absolute failsafe. You could define Areas within the level, and the when the player reaches the end of the level, it's a checkpoint, and it saves their weapons, ammo, health, and which limbs are missing. When the save is loaded, it can restore all of that data and kill any enemies that spawn within previous areas.

I have to say, I would be happy to spend as much as an hour if there was saving.

I'd go for an hour and a half, Also I rarely ever get crashes with vanilla content.

Okay, thanks for all your responses.

At the moment I'm thinking I'll make a shorter half-hour version with some of the features I have planned, to gauge interest.
Furthermore, if I can put that together, then I'll have a better idea of how difficult it'll be to get the final version together. Half-hour version probably won't have saving, but you never know.

I really like the idea of character building, but I'm not really digging the idea of Demon Souls style death penalties.

Well, if you do need a checkpoint system, you can see a simple implementation (that may not suit all of your needs) in Sweet Dreams.