I find if I stop a game I don't feel like picking it up again, I just want to start a new one, so I often try to play a game all at once, but I like epic speed and the games take me 4-8 hours. I also feel like I want to play one every day

It's affecting my grades. Have any of you figured out how to deal with this?
Yes. You do your job/school, then you play civ. If necessary, do both school work and civ faster. 4 is on the upper end of my game times, even on epic, so 1 sitting is doable

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My grades aren't perfect but I put what I'm willing to put into them, unaffected by civ (prior to being here I found other ways to screw around...), but I'll take my 3.83 since I'm in grad school. I do miss the high school days of inflated well-over 4.0 GPAs though.
Don't neglect sleep IMO (it catches up, although easier if you're in your teens than twenties, twenties than 30's, and so on), and you can't neglect real-life obligations long. They catch up, and man does it hurt if they do. Just avoid that problem in the first place. Raising one's efficiency in school can be a big help.
For example, I used to take a couple hours to read 30 ish pages and write a few pages based on the reading, but now that's a weekly assignment that I do for about an hour at work the day of class. 5000 word papers are a weekend foray (although a bit annoying to do TBH, especially if you need 10+ sources).
In order to do that, you need to practice speed reading + comprehension, typing (which we do get some practice with here), and the ability to find useful information rapidly. If math is involved, work out whatever process in your head that is effective so you can repeat it faster without screwing up. Working on memorization can help a ton. Of course approaches vary by subject too.
Edit:
1. Stop game.
2. Go outside.
3. Find girlfriend.
4. ???
5. Profit - no just kidding...
5b. Initiate girlfriend to Civ IV.
If you can consistently find a way such that girlfriend = profit, go collect your Nobel prize. Unless you mean it's the girl that profits...but those aren't "girlfriends", strictly speaking!