Stuck over tree climbs

My three-year-old male cat has started climbing trees in my garden. I am worried he will end up stuck at the top of the tree. How long is it reasonable to leave him before getting help, and how? What is the best way to encourage a cat to come down?

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Cats stuck in trees is a common occurrence and reason for calling animal rescue societies or the fire services. But in my career I have never been asked to attend an incident or recall treating a severely injured cat that had fallen.