I've always had pretty sensitive ears, too, and in my experience it starts about 40-90 minutes before landing on longer (intercontinental and some transcontinental) flights, but about 20-50 minutes before landing on shorter (midwest to the coast) flights.

Crying definitely helps babies, but so does sucking, so in addition to feeding the baby a pacifier can work wonders. For older kids and adults, decongestant, chewing gum and sipping water are good. And warm things (gentle heating pads or a pair of styrofoam cups with hot-water-soaked-napkins stuffed in the bottom) can be very soothing as well.