I asked a lady and her father to marry me and they both agreed. Only now, the father says he is arranging the wedding and I'll be notified when it happens, many week have passed since then. Is it supposed to take this long or is something broken? How much time does it usually take?
The part where "they let you know when the wedding feast coming up" is broken. For me, basically I camp out at the city where the wedding suppose to take place. I just keep poping in and out until I see the "feast going on" option. I select go to the feast and talk to her brother, then all the sudden he say "welcome to the wedding feast". I did this twice before I get that message. Hope this help.
Did you pay her guardian a load of money as a security for her? Maybe you should ask for it returned if so. Just try another lady and cut your losses. The only non-bug related reason I can think of for what you have described is that your Kingdom might be going on continuous campaigns. I noticed that it only took about 1-2 weeks for the wedding feast to occur in my game. The delay was because there was an offensive campaign going on. Once it ended though, I was notified to appear at the feast pretty promptly.