How long can I drive after my "need gas" icon shows? on 2008 Suzuki Forenza

After my "need gas" icon turns on, how many miles can I drive?

Asked by for the 2008 Suzuki Forenza
Check the owner's manual. It will tell you the capacity of the tank and the volume of the reserve.

The owners manual does not say how many gallons remain after the light goes on; that's why I asked the question.
I checked a manual and that doesn't say it either. Here is how you can figure it out.
The tank capacity is 14.5 US gal (55.0 L), fill it up all the way, drive it until the light comes on, note the mileage and do a mile/gallon calculation, refill it, note how many gallons of fuel it takes. Now you know how many gallons remained in the tank when the light came on and how many miles you can drive with it based on your gas-mileage.

Thank you; is there any other way except to wait until the light goes on?
You can try to write an e-mail to Suzuki's costumer service and ask them. They might know.