How do you get the wedding ceremony music timing right?
At rehearsal, have the guitar player play as he/she would for the wedding party and find an appropriate stopping point after they are all in their spots. It won’t hurt anything if he/she has to play a few seconds after everyone is settled, and that may help the photographer. Plus everyone has a chance to catch their breath in anticipation of you. It actually may take 3 minutes for the wedding party to enter and get set, especially if they are walking slowly, and if you have the groomsmen come in one by one. It could definitely take 3 minutes if the bridal party doesn’t start right away when the music does or if there are any long gaps in between the members walking down the aisle. When you come down the aisle the same thing will happen. Practice the speed at which you will walk (based on the tempo of the song) and the guitar player will be able to find a good spot (and a couple of backups) for stopping.
My friend recently got married and for her ceremony music she picked a part of a song she loved and we practiced to see how long it would take the bridesmaids to walk down the isle until we got it just right, then on the wedding day all we did was slowly start turning down the music to make it fade out before her processional started. We did the same thing for her processional and it turned out great.