It all depends on the level you are playing, the kind of map, and ultimately how the AIs are doing. It is not possible to fix any years at which you could say what is to happen.
I usually start with a particular victory condition in mind, and work toward that to the exclusion of others. E.g., diplomacy becomes a secondary concern if I aim for a domination or conquest victory (though diplomacy remains important even for those conditions, at least early on). Thus, there is usually no decision to be made.
Now that I have dodged the question for two paragraphs, I can say that, on the rare occasions when I just go with the flow, the decision is usually made for me by the start of the industrial age. At that point, the game is usually all but won, and the only question is how to escape as quickly as possible.
In my mind, conquest is for very early victories on small maps. In that case, you don't get to the industrial age.
20k cultural victory requires planning from the very start of the game. The decision must be made before you start.
Of the remaining conditions, domination happens the earliest in years, and probably leads to the most points, but all that war takes a lot of time. If I'm not at e.g. 40-45% of land mass by the beginning of the industrial age, I usually choose not to do that.
"100k" cultural victory is, in practice, almost the same as domination, in that the way to do it is to have a lot of cities (with lots of culture in them). It is actually my favorite victory condition, so if I have 1/3 of the culture I need by the beginning of the industrial age, and a solid cultural lead over my opponents, I go for it.
Diplomatic victory is one of the easier conditions, if you haven't thumped too many AIs. If my land area and culture are deficient at the beginning of the industrial age, and if only one or two AIs hate my guts, then I plan to buy off the rest and use diplo for an early exit.
Space race is the victory condition of last resort for me. It's what I use when I have spent a great deal of time in badly chosen or unlucky wars. Everybody hates me, but I haven't captured enough land to take an early domination win. In that case, I just wage desultory wars to cripple any AI efforts to keep up technologically, and build the spaceship wherever. In my belief, it is the easiest victory condition (except in competition!).
The point is that you can't really put numbers on any of those things, and on the other hand, domination, cultural, and space victories are always available to a patient person. The only question is how long you are willing to play for the condition you want.
