Long pieces may involve several mood changes, and it's lovely not to disrupt these with applause. Quite often a classical piece has several sections, each with its own ending, and it can be hard to tell which ending is the final ending, the one you're supposed to clap for. When in doubt, simply wait until lots of other people are clapping. In other times and places, audiences clapped throughout the music. Mozart, for instance, was proud to report in a letter to his father that there had been wild applause during his latest symphony.
Snobs are only too ready to sneer at people whose enthusiasm results in mistimed applause.
Such snobbery should be pitied but ignored. As a performer, I'd much rather play for overzealous applauders than for snobs. Sounds that Gets in the Way Talking. Squeaking a chair. Opening a purse.