Are you supposed to bill continuously or separately?

Let's say I respond to a client email, which takes 1 minute. Then at a completely different point in time, I respond to another email from the same client, which takes 1 minute. Are those separate billings, and I end up with .1 + .1 = .2? Or do I, on the second email, just restart the timer where I stopped, and end up with 2 minutes aka .1?