When the light comes on, if you drive several hundred more miles (approx) an entire 2.5 gal jug of DEF will fit without overflowing. Terra Cair Ultra Pure DEF (sold at Tractor Supply and other places) has a fill spout that fits better than the Blue DEF typically sold at Wally World. DEF is mixed in at approximately 50:1 ratio to fuel being consumed, on average, so # miles before light comes on depends on a number of factors, but mainly how much fuel is being used per mile. That varies with terrain, whether towing, speed, etc.
Hope this helps,
(former owner, went back to Class A)
(but enjoyed our brief 'experiment' in downsizing!)
'16 Forester MBS 2401R
Hellwig 7254, TireMinder A1A & Sumo Equipped