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Don't think we've heard anything about time limits so far, though, yes, they are a possibility.
And we've yet to see what kind of consequences the surrender option has, if any. I remember Jahlon saying that there's some monetary ones, but I don't think I've seen a source for that.
And again, from everything I've seen so far - it doesn't seem that we can set objectives. It seems that we can set "stakes", which probably dictates the cost for your declaration and probably the cost of surrender for your target. But the objectives themselves seem to be chosen by the system.
Theoretically, people will extrapolate the value of different guild actions and will then track them in other guilds to figure out what kind of objective they'll get if they set particular stakes for the war.