It obviously varies by bike and how you ride it.
I would not rely on getting further than 20 miles maximum.
Firstly the advice, if you haven’t got a fuel gauge, is to reset your odometer everytime you fill up with fuel.
When the bike goes onto reserve, note the mileage for future reference.
When you fill up you can then work out your fuel consumption, and from the capacity of the tank you can calculate how for you are likely to get on a full tank and reserve.
It can be extremely dangerous suddenly running out of fuel.
If your bike has a reserve tap rather than a light, it can also be very difficult to switch on the move.