Bit hard to say but, a minimum would be a ban from all sporting grounds (think thats how it works - could be lifetime could be for a set number of years. Minimum is usually 12 months).
If the headbutt could be proved as with intent and caused bodily injury then it would be GBH with intent - that would be 4-5 years (based on an existing criminal record). If the charge was less then the scale of the punishment would reduce similary.