A criminal record is for life. (i.e. if you were to appear before a court in 50 years time, the theft offence would still be listed on your record. Similarly, it will always appear on police records).
However, most offences become 'spent' after a certain period of time. (The relevant period for an offence dealt with through community service order is
5 years or 2.5 years if you were under 18 on the date of conviction). After this period, you don't have to declare the offence when applying for employment or, for example, filling in an insurance application form. (Normally, if you lie when making these types of applications, you could be prosecuted for 'seeking to gain a pecuniary advantage by deception'. However, if your convictions are spent, it's legal to write 'No' when you're asked 'Have you ever been convicted of a criminal offence?' ).
There are a few important exceptions:
(i) if you're convicted of another offence before the first offence becomes spent, the first offence won't become spent until
both convictions are spent ;
(ii) some types of employment are exempt from the provisions of the legislation about spent convictions. These are generally jobs working in, or alongside, the caring professions (e.g. teacher, school caretaker, nurse, hospital porter, care assistant). When applying for these jobs, convictions are never regarded as spent ;
(iii) convictions never become spent when the laws of overseas governments apply. For example, the USA requires all visitors with a criminal conviction to apply for a visa, irrespective of the time elapsed since the conviction.
http://www.lawontheweb.co.uk/rehabact.htm
Chris