It should last as long as a regular open-borders treaty lasts.
Yea the diplomatic boost from restoring someone is SOO small it is basically non-existent. It sucks
Thanks. This could put me in a pot of trouble. I'm currently playing prince on the huge earth map, as Rome. Mongolia has/had most of eastern europe and asia. I OB'd Perisa to get to Russia, went back and took Sparta so I don't have to rely on Persia, then ventured east again, and now the bulk of my forces have a few Russian cities behind them.
My happiness is high enough that I could keep a city or two as I go, but the real issue is Egypt south of me, which now controls most of Africa. If they attack, it will get interesting very fast.