As Sweetheart78 states,the maximum sentence is life imprisonment. The actual sentencing guidelines which judges must refer to are on page 13 of this document:
http://sentencingcoun...person_accessible.pdf
It should be noted that the sentences in that document refer to a first-time offender who is convicted after a trial. An early guilty plea can see the sentence cut by one third but previous convictions can push sentencing higher.
It should also be noted that, instead of the sentences imposed there, an 'IPP' (Indeterminate Public Protection) sentence could be passed. It's very similar to a life sentence in that a minimum term is set by the judge. The offender can't be considered for release until that term has expired but there's still no guarantee that he'd be released. (His release would only happen when it was considered safe to let him back on the streets again).
Chris