Really, when I shot weddings, I changed the film when I needed to. I knew the progression of events fairly well and could plan ahead and change film when there were low spots. I would shoot a roll pre ceremony. I would shoot a roll coming down the aisle. I would shoot a roll during the ceremony. I would shoot a roll coming up the aisle. I would shoot a roll about the church after ceremony. I would shoot a roll of toast, cake (not the cutting), some candids. I never shot the announcement. The clients always bought the shots coming back up the aisle so announcement was a waste. The I would shoot the dances. The last dance I would shoot early on so I had time to change for the cake cutting and the bouquet/garter fiascos.
Just plan ahead and time your shooting as best you can.
Of course, in a perfect world that would work everytime. It was successfull enough, though.