There is a cross feed between the two, so fuel drains from both tanks at the same time. In normal operation, the reading on both gauges (main and aux) should reduce together as fuel is used up. As has already been mentioned, the aux tank reaches empty first. Once this happens, the reading on the "main" gauge will then start to drop much faster as fuel is being supplied only from the one tank. The aircraft will continue to fly with the aux empty but it's definitely time to start thinking about getting down !
Throughout all of this the rate of fuel usage should be around 15 gph, so you can monitor it that way. Just watch out for the faster progress of the "main" needle in that last 1/4 of a tank.
There used to be a fault on the R22 where the crossfeed tubing could get kinked or blocked, so if you saw the main tank getting used up but a constant reading on the aux that would be the time to get worried. This was fixed several years ago however.
What was your "scenario" ?