Judges are required to work from a 'starting point' and then take into account aggravating or mitigating factors to move the sentence 'up' or 'down' but still (unless there are very exceptional circumstances) within a fixed range.
For a particularly grave injury or disfigurement resulting from a pre-meditated assault where a weapon has been used, the starting point is 3 years custody (with a possible range of 2 to 4 years)
For a pre-meditated assault where a weapon has been used or an other assault where particularly grave injury results, or where a weapon has been used, the starting point is 18 months custody (with a possible range of 12 months to 3 years).
For a pre-meditated assault where no weapon has been used, the starting point is 36 weeks custody (with a possible range of 24 weeks to 18 months)
For any other assault, where no weapon has been used, the starting point is 24 weeks custody (with a possible range of a community order up to 36 weeks custody).
Those sentences apply to first time offenders who're convicted after a trial. An early guilty plea could result in a reduction of up to a third of those sentence periods.
The foregoing assumes that the charge is 'Section 20' (GBH) and not the far more serious 'Section 18' (GBH with intent). They also assume that the victim was not under 15 years old and that the assault was not racially aggravated.
Chris