The maximum penalty for a Section 20 offence is 5 years imprisonment. With his previous record (and the aggravating factor of the burglary charge), it's likely that the court would consider the maximum sentence but possibly with a reduction for an early guilty plea.
My guess is that if your boyfriend pleads not guilty, but is convicted, the sentence will be between 4 and 5 years. If he pleads guilty at the earliest opportunity, he might get a year or so less.
It's also possible, if the court believes that your boyfriend presents a danger to the public, that an IPP ('in the interests of public protection') might be passed. This is similar to a life sentence in that the court sets a minimum period for imprisonment but the offender is not released (if ever) until he's proved that it's safe for this to happen. If an IPP was passed, it's likely that the minimum tariff would be around 4 years.
To summarize:
If your boyfriend pleads guilty as soon as possible, he'll probably actually be 'banged up' for around 1� to 2 years.
If he pleads not guilty, but is convicted, he'll probably actually be 'inside' for 2 to 2� years.
If he's given an IPP, he'll probably be in prison for at least 4 years, possibly much longer.
Chris