In my experience, oversteep will cause bitter taste especially when brewing tea by gaiwan. I usually use gaiwan to brew tea so I think time is quite important. Different tea need different time to brew. I think 3-5 minutes is for teapot brew, this time will be a little longer for me. For green tea I use glass cup to brew, I usually brew for 2-3 minutes. For white tea I use gaiwan to brew less than 1 minute and do not cover the lid because it may badly stew the tea. Oolong and pu-erh is best to use Yixing teapot to brew.
The time for brewing tea is actually according to your taste, stronger more time, light less time. I usually try with different time to see which result the best taste for me.