I just turned down Poland's Declaration of Friendship offer, because Casimir was at war with Egypt. The next turn, he and Egypt made peace. I waited a turn to see if there were any immediate denunciations and seeing none, I'd like to be friends with Poland. Poland still feels "friendly" to me. But after turning down a DoF, there's waiting period before you can offer one to the same civ. I've seen it before, but I've never seen it documented. The option to offer one just doesn't appear (rather than being grayed out, with a tooltip saying how long you have to wait, like with city-state options). I play Epic, so I guess my answer is 1.5 times the number of turns in a Standard pace game. But I don't know how long that is.