I think balancing victories condition based on in-game length (turns) isn't a good approach. It would be better if we balance it based on the amount of work needed (aka real-time game length, not in-game turns).
For example victories that constantly give you more things to do like domination (manage warfare) or science (both research and production of new building/units that come with new techs) should come earlier than victories with less things to do like culture or diplomatic (mostly just passing turns while repeating the same thing/waiting for specific timing like GM bomb or next congress), and the slowest one would be the most boring ones (time).
Reason being the more active victories should give faster result in perfect condition, but since they're usually harder to achieve they will get delayed and end up with similar timing of other easier but slower victories (plus civ aiming for easier but slower victories can spend more time trying to sabotage civ with earlier victory instead of sitting tight passing turn)
Based on score/time victory at turn 500 standard speed, I would say if we can balance culture/diplomatic/science victories around 450/425/400 it would work best. Domination would need the new proposal passed, but aiming for 375 would be reasonable. Reminder this is perfect condition (when you're running away), the actual turns to get the corresponding victories would get delayed and generally would happen around the same time for all victories (450-500).