Thanks for your question
Currently (from 06/04/2013) this sits at £9440 a year, which breaks down to £786.66 a month/£181.53 a week.
If you are employed, the employer allows you to earn either the monthly or weekly amount (depending on how often you get paid) each time you are paid (rather than give you the full allowance for the year in one go) and then deducts tax and National Insurance from any excess each time you are paid.
But if you are self employed, then the annual £9440 is allocated against your net income (income less expenses) in one go,. and if you have income left over and above this amount, then tax (and National Insurance) is paid on the remaining amount, and paid to the tax office no later than the 31st Jan following the end of that tax year.
Please feel free to seek clarification on any aspect of my answer, but if in the meantime you could rate the level of service received, it would be appreciated as this ensures I am paid. And any feedback and/or bonus are always welcome.