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Doesn't the game completely cut you off at the turn limit? I can't even remember if I have ever played a game to the turn limit to check but if it cuts you off then 330/500/750/1500 are going to be absolute maximums for turns you can use.
Ok, now i can answer this precisely.
The game _might_ cut you off, but if you do the following, then it won't do that:
1. disable all victory types except time victory at the game's start;
2. win the game (by having highest score at the "last" turn);
3. press "one more turn" button (which, it seems, is active only if you did win (and did not lose to some AI) - thus p.2);
4. go on playing.
Note, i suspect that the same might be possible after any kind of win, except domination - but i didn't check it myself.
Source: 1st-hand experience. My current game was created this way exactly (p.1); i won the game (had highest score at the turn 750 - it's epic speed); did p.3; at the moment, it is turn 1050, year 2200, and there is no sign it'd ever end. Someone said somewhere that "one more turn" only goes to 1000 turns - apparently, this is not the case; at least, not on Epic speed. If/when i'd go past 1500 turns, i'll try to drop an update into this topic.
Here's the catch though. I knew unit maintenance costs go up with time - but i did not expect it grows up that quickly. At this point in my current game, i am paying 17+ coins of maintenance per unit. I have 104 units in total. So it's ~1800 gpt to keep 'em units up. Unit maintenance cost total nearly doubled during last ~300 turns. So far i still have some +800 or so gpt, but it doesn't feel too promising. Considering current balance, reserves (~400k) and potential for more gpt, i'd say i have at very least 600 more turns before i'd face danger of bankruptcy - possibly 800+ turns. But once there, it's either massive cut to army size, or hugely negative balance "forever". Apart from science hit (which i can live with, for sure - future tech is useless anyways, since i already won and score is not really needed) and inability to buy anything (also tolerable - have everything) - are there any other undesirable effects of having huge debt?