The guage on our '85 GL never goes to the top.  The tank can be over full and the guage will read a little over 3/4.  I was told the problem might be the "voltage regulating transistor" on the back of the guages.  I changed the transistor but the guage stayed the same.  I guess I have a problem with the sender in the fuel tank.  If I ever drop the tank I will replace the sender.
To answer your original question I try and fill up every 300 km (about 180 miles) or so when we are on a trip depending on wind, hills etc.   There is usually room in the tank for about 50 liters (13 us galons) of fuel (the tank holds 60 liters)   After 3 hours I am usually ready for a short break from driving.
One other small point.  If the tank is looking empty a the end of the day and we are ready to camp then I wait until the next morning to fill up so we don't have to smell gas while we are camping (cooking, eating, sleeping etc).
George
'85 Westfalia