Approx 4:00PM

Tea time is a British invention, that sets aside some time for tea in the afternoon. Compare with a Spanish siesta or a stoner's 4:20, or possibly even Beer:30.

Says Charles Oliver in Dinner at Buckingham Palace:
The ritual of the English tea-time was brought to perfection by the late Queen Mary, for whom it was the favorite time of day. Everything had to be fully ready by 4 P.M. punctually, with sandwiches, cakes and biscuits invitingly set out on gleaming silver dishes upon a smoothly-running trolley. The teapot, cream jug, hot-water jug and sugar bowl were always the same antique silver service which had been a favorite of Queen Victoria...{Later} Queen Mary would take over and meticulously measure out her favorite Indian tea from a jade tea-caddy she kept locked in a cupboard. Then she would pour on the boiling water and complete the tea-making ritual by snuffing out the spirit stove before sitting back for the footmen to pour tea and hand round sandwiches and cakes. But before Queen Mary gave the signal for this to begin she would always let exactly three minutes elapse from the moment she poured hot water on the tea leaves so that the tea would be perfectly brewed.

Another source, the American*British Dictionary says that that tea time is sometime between 4 and 5pm, and is either the last meal of the day -- dinner -- or a short break period between lunch and a later dinner at 7-8pm.

The American*British Dictionary can be found at
"Dinner at Buckingham Palace" a 1970s book, edited by Richard Warwick. I assume that Queen Mary, then, is Mary, Queen of Scots, and not Bloody Mary I.