Is tea that has been out on air for long dangerous to drink?
As long as you haven't added something that will dramatically increase the spoilage time (milk, or sugar), it should follow roughly the same rules as coffee. Most citations I've seen say the taste begins to go off at around 8 hours, which is a good indicator of spoilage. It's going to taste yucky if you forget and leave the teabag in it as well, but that's just over-steeping.