I have mentioned before that a tea company Suki tea gave me there tea bible and inside it tells the reader how to judge your tea is ready by the colour it produces. This method was also mentioned to me by my tea tasters, we should be keeping an eye out for the water changing colour.
To do this with a glass cup would be easy as you can see the tea clearly and easily. From my pictures I posted up about my form you can see a few cups have a white interior and this white interior will show the colour of the tea a lot better.
For time the user will judge the tea and its colour to tell when the tea is ready instead of using a timer.