As Redcrx indicates, you've not really given us much to go on.
Things are further confused because you state that you've been charged with Common Assault, which under most circumstances can only be heard in a Magistrates' court and not before the Crown court. (The only exceptions I'm aware of relate to circumstances where you're also charged with an offence which can, or must, be heard in the Crown court. Under such circumstances it ought to be the more serious charge which should be worrying you).
The maximum penalty for Common Assault is 6 months imprisonment. Given your previous record it's possible that the court might consider imposing that maximum sentence. However, the exact circumstances of the assault will obviously be extremely important. (Simply pushing someone, or spitting at them, amounts to common assault but it's unlikely that a court would regard such actions as seriously as repeatedly punching someone). Additionally, an early guilty plea, together with any relevant mitigating circumstances (such as evidence of provocation) might get the sentence reduced.
On the very limited (and somewhat confusing) information that you've provided, my (very) rough guess would be a sentence of around 4 months imprisonment.
Chris