Unless there are really exceptional circumstances (which a judge must explain in open court), judges are obliged to stick to the sentences in this document:
http://www.sentencing...inst-the%20person.pdf
(See the table on page 13)
The sentences in that table apply to a first-time offender convicted after a trial. An early guilty plea can see the sentence reduced (by one third in the case of a custodial sentence).
Kicking a victim, particularly to a vulnerable part of the body (such as the head) can be counted as equivalent to 'use of a weapon'. (See Section 22 on page 6). So the judge is likely to view the assault as falling within the category of "Other assault where particularly grave injury results or a weapon has been used". Taking the lowest point of the relevant sentencing range gives a sentence of 12 months, but the one third discount cuts that to 8 months. Only half of the nominal sentence is spent in prison, so expect to be 'out of circulation' for about 4 months, or possibly slightly longer.
Chris