A lot of tea brands here in the US have geared themselves toward making sweet tea, which bears little resemblance to what they consider "tea" across the pond. I pour 3/4 cup of sugar into a gallon of warm/hot water (hot from the tap, not boiled), stir to dissolve the sugar, then drop in a tea bag (various brands*). I then put the container into the fridge to let it turn cold (usually 2-3 hours). I can tell how "done" it is by how dark the tea becomes, but I must confess (sorry, Sarah) I can't drink sweet tea warm/hot. It has to be cold, though I don't like it over ice because the melting ice dilutes the taste. It has to start out hot (or very warm) to brew properly, though; if I did my "lazy version" of sweet tea with cold water, the sugar would sink to the bottom and not dissolve properly, and when you're using THAT much sugar you need all the help you can get.
* Normally I use whatever tea bags are on sale, though I rarely buy brands I'm not familiar with. On a whim I bought PG Tips, a brand not very well-known here but bills itself as "England's Favorite Tea". I worried that it would not brew a gallon the way it brews per cup, but it turned out pretty much the same as all the other brands I've used.