I have a real hard time guaging other Civs thresh hold for staying friendly. In the past I've decided against declaring war when a Civ asks and it is not in my best interest to ruin relations with the target Civ. Doing so over time seems to degrage relations with that friendly Civ for not declaring war. I do know if you ask to wait 10turns and then decide against it, you will receive a penalty. What has happened in my last couple of games though is I've went ahead and declared war when a Civ asks me to, then waited until I can negotiate peace. Multiple times this ends with them trading me a city for peace and then a few turns after that the Civ asks for DoF even if I never attacked them. Other experiences are being completely friendy to a Civ, giving free gifts, multiple trades, and helping them in a war conflict and having a complete list of green actions on the diplomacy window. One bad vote in congress and they denounce me completely and remain guarded the rest of the game.