Assume a long and large-scale war (say, 30 years long) was fought between two large interstellar polities (one about a sector in size, the other several sectors in size). Ceasefire was eventually agreed on, and, eventually, a proper armistice treaty and trade agreements. How long until both states will demobilize large segments of their armies? Until their economies will gear towards peacetime production? Until politics will stabilize in areas conquered by one from the other?
Ummm - roll 3d6, multiply by 2d6, then subtract 4d6??
That is an awfully broad question. Actually, that's three awfully broad questions.
"How long until both states will demobilize large segments of their armies?"
Anywhere from within weeks to never. The fact that there are still two sides after a 30 year war between one sector and a polity covering several sectors speaks to factors not mentioned that have somehow kept that one sector alive or, dare I say, given them the victory. Who won? What triggered the war in the first place? How much do they really hate each other? How many times has this happened before, and are the people generally tired of it or are they angry and resentful at the outcome and biding their time for a rematch?
"Until their economies will gear towards peacetime production?"
Anywhere from months to never; see above for the reasons. And, if they do, the effort could trigger an economic collapse, or it might trigger a boom. And an imbalanced outcome - a boom on one side with a collapse or malaise on the other - could set the stage for the impoverished side to start gearing toward war again, if for no other reason than to distract the unhappy populus.
"Until politics will stabilize in areas conquered by one from the other?"
Anywhere from a few years to never - especially if a follow-up war occurs. Even if it doesn't, it could turn into one of those Northern Ireland situations.
Ultimately, I think the answers are variable enough that you can choose the one that suits you and justify it as easily as any of the alternatives.