I put them in before take-off and left them in until the seat-belt lights went off, then put them back in about 45 minutes before landing (descent is always more slow and gradual). My ears 'popped' on their own without me having to do any yawning or chewing, but the plugs made my ears feel sore and I'm not sure if I would have preferred to go without them and just stick to my usual packet of chewing gum - however, they were a bit of a godsend on the way out when we had a screaming baby in front of us (screaming for the entire 4½ hour flight...)
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