The information provided by dizmouk is not correct.
Magistrates can only impose a custodial sentence of more than six months if they are sentencing two or more “either way” offences. In those circumstances their powers are limited to one year (two sentences of six months to be served consecutively) regardless of the number of offences being sentenced.
However, Criminal Damage and Common Assault/Assault by Beating are “summary only” offences. They can only be dealt with in the Magistrates’ court and cannot be passed to the Crown Court for sentencing no matter how many offences are beating sentenced or the seriousness of those offences. As a result the maximum sentence available for these incidents when dealt together is six months. The partner of ct will apparently be pleading guilty. If he does a discount of one third must be allowed, making the maximum four months. All prisoners are released when they have served half of their sentence. This release is automatic and unconditional. In addition to this, current “Home Detention Curfew” arrangements make prisoners serving sentences of between four and twelve months eligible for release when they have served one quarter of their sentence, meaning they will be released after one month. So even if ct’s partner is sentenced to the maximum (by no means a certainty) he will serve just four weeks.