My mildly modified 21 normally does about 120 miles from full to reserve, I think it did about 140 when it was stock. Factor in fully synthetic at £10 plus a litre and its not the cheapest way to commute but its definetely more fun
My daily commute mileage is about 50 in the car and about 80 on the nsr

Fuel consumption?... I'm lucky to get my MC21 out of the hign teens when I use it!

So a tank full should last you about a year then Andy!