Wow, I feel so special that you want to try my tea!!
I guess I just look at it, see the color, and know when it's dark enough. I've never had it get "overdone" even when I left it out for hours. You can really make this tea anywhere, even in the fridge, but since I think it's a great drink for when it's warm and sunny I usually only do it outside on a sunny day. The less sun the longer it takes.
I'm sure you could use any type of tea. You just might have to experiment with different types and different lengths of time. I am boring so I use the plain ol' Lipton tea bags, although I am using decaffinated right now since I'm pregers. My mom uses green tea.