Tea time is any time we want a cuppa.
If you mean a meal called tea, then afternoon tea, which is supposed to be a snack, is around 4 pm. This only exists in expensive hotels, because tourists have heard of it, and in upper class houses where people can afford to have the time off. It was invented by the Duchess of Bedford when dinner was fashionably getting later and later, and she felt like a snack of sandwiches, cakes and tea to keep her going until dinner. Then she started asking friends round for tea and the idea spread. Strictly for "ladies of leisure" - you can hardly do that routinely if you're at work.
The only other place I've seen it is at my former church, where one of the ladies would do it for fund-raising purposes. "Come to Tea" at her house, and she'd have a big baking day and invite everyone round mid-afternoon on Saturday. No chance of everyone getting a seat so it was more of a sociable buffet with everyone standing around chatting with cups of tea and something to nibble. Very like a royal garden party on a small scale. (The difference at Buckingham Palace is it's a much bigger event, there is a dress code, and the whole thing is in the enormous garden with tea and food served from the tea tents.)
Tea is also a regional word and class word for the evening meal, so it's whenever you have that. Growing up in a family of working-class origins, I grew up with breakfast, dinner and tea.