With most benefits your right to receive them doesn't disappear until you've worked over 16 hours but the actual amount you receive in those benefits diminishes depending upon how much you earn.
For example, with Job Seeker's Allowance you lose £5 less than your earnings from your benefit. As an example, let's say that you're getting £65 per week in JSA. If you work for 8 hours at £6 per hour, you'll earn £48. So £43 (i.e. £5 less than what you've earned) will be deducted from your JSA, leaving you with £22.
That means that working for anything up to £70 worth of pay can never increase the actual money in your bank account by more than £5. If, as is likely, it costs you more than £5 in fuel or fares to get to that work you'll be worse off than if you simply stayed at home.
Chris
PS: We're all waiting to hear, on your other thread, which of the 'classics' you've decided to read ;-)