When we've eaten at Le Cellier, it's usually taken a little over an hour. However, we are a party of 4 and have early dinner times, so that may play a part in how quick it is for us. I must emphasize that we've never felt rushed. As one of the most popular restaurants at WDW, I think they just have their act together!
In any event, plan for more time. If your luck is anything like mine, when you need everything to go well and quickly because you have another event to get to, everything will go wrong and take twice as long!