The nature of offence is irrelevant in terms of when it becomes spent,it's the actual sentence that matters.
Recent changes have reduced these periods and are as follows:
Any sentence (includes suspended) over 4yrs is never spent.
Sentence of 30mths - 4yrs is spent after 7yrs from completion of sentence,not how long actually served,ie if sentenced to 4yrs and served only 2 it would still take 11 yrs to be spent.
6mthys - 30mths is 4yrs from end of sentence.
less than 6mths is 2yrs + sentence .
CRB is now called Disclosure and Barring Service and what they disclose is dependant on the employers level of entitlement.
As already said it's basically places like schools and care homes with kids and vulnerable people that have a right to access ALL your previous convictions AND police records of arrests etc - even if no further action taken or cautioned.If they want a check they'll arrange it and pay for it.Shops,offices etc are only entitled to a basic check and ONLY unspent convictions will be disclosed.Again I'd leave them to do if they feel it's necessary - and they almost certainly won't. Hope this makes sense and is of some use.