Just go and ask your solicitor, he or she will be able to tell you whether or not it is spent. It depends on the type of sentence you were given...e.g a crime resulting in a prison sentence takes longer to be spent than a probation order, a probation order longer than a simple fine etc.
As dzug pointed out, for some jobs it is never spent....examples include some types of civil service jobs (especially those involving law enforcement), or any type of work involving children or vulnerable adults. Such a record does not mean you will automatically be disqualified from the application, but they will of course take it into account. Again, the same does apply for visa and immigration purposes for certain countries. Some will make more difference than others, for example if someone has a drug conviction it is far more likely that they will get stopped by customs at an airport than someone whose record is clean in that regard, but a theft conviction is unlikely to result in that kind of thing occuring.