it's different for everyone and for each situation. my wife needs zero foreplay, she's always ready and willing to go at any time or it's a flat no and "i'm not in the mood but i'll suck you off it you want". even sometimes when i'm not in the mood, i'll just tell her to ride it til she's satisfied cause the dick will still rise to the occasion even if i'm not willing to put in the work, hehe.
i used to be one of those guys that would spend 20-30 minutes on foreplay to make sure she was super-wet and horned up before i gave her the D but now i realize that's all just hollywood-BS and each woman/man is different so i just come out and ask what they like and go from there.