In general, where can one find information about the quantity of fuel that is left in the vehicle's tank when the warning light turns on?
More specifically, in a Suzuki Swift GLX (auto) 2010 model?
Look up in your owner's manual how capacity your fuel tank has. Next time you fuel up, take note of the difference. In my car it's roughly 2 gallons (7.5 liters). However, realize that modern cars use fuel as a coolant for the fuel pump and running the tank dry often may damage the fuel pump over time.
Look up in your owner's manual and it should tell you how much fuel is left when the light comes on. You can always confirm it by following Parker's answer too.