As far as the visa question is concerned, this has been answered in the Travel section.
However, here's some further information about 'spent' convictions:
1. A criminal record is for life. Your partner's conviction will always be on the records and available to the police and the courts if he gets into any further trouble. (it will also always appear if he applies for employment which requires a CRB check) ;
2. However, for most purposes, a conviction resulting in an 8 month prison sentence is regarded as 'spent' after 10 years (or 5 years if the offender was under 17 at the time of the conviction). So, if your partner applies for employment or insurance and he's asked 'Do you have any criminal convictions' he must currently answer 'Yes' (otherwise he's committing a further criminal offence of 'seeking to gain a pecuniary advantage by deception'). Once the 10 years is up (and assumingthat he's not convicted of any other offence), he'll be able to answer 'No' to that question on all insurance forms and most employment forms.
3. Applications for some types of employment are exempted from the rules on 'spent' convictions. (These are basically jobs working with, or alongside, children and vulnerable adults, together with jobs involved in the 'administration of justice'). If your partner was to apply for such a job, he'd always have to declare the conviction.
4. The rehabilitation of Offenders Act only applies to the UK. As far as other governments are concerned (e.g. when applying for visas), convictions never become 'spent' and must always be declared.
http://www.lawontheweb.co.uk/rehabact.htm
Chris