You can calculate the time by factoring in the extra steps required after the injectors are removed. So you've got the labor time for injector R&R, then you need to drain and refill the coolant, my puller setup I made requires that the rocker arms be removed, so I factor that in and the time to pop 1 cup out and clean the loc tite out of the head, and bang a new one in.....times 8.
Obviously you want to be competitive and fair to your customer, but you need to look out for yourself also. If the customer baulks at a given quote, tell them how much it would cost them if you removed the heads and did it on the bench. That usually squashes any arguments.