From the R22 POH, caution/warning lights section...
"LOW FUEL Indicates approximately one gallon of usable fuel remaining. The engine will run out of fuel after five minutes at cruise power."
As a low hours pilot, I did some emergency training with a very experienced instructor - 20,000 hours or so. He told me that in the event of the low fuel light coming on, land ASAP, as from 2000 ft, at a steady descent rate of 500 ft/min..... well, you can do the maths too.
Experienced pilots can work out other ways of doing it of course, but it seems obvious from this what you should teach students.