In The United States, What Is The Percentage From Taxes Taken Off Your Cheques?

As a general figure it's about 15% to Feds and 5% to State, or 20% total.  If you get any lump sums paid unexpectedly the recommendation is that you pay 20% of the amount immediately as tax to the federal government.  In some states you'd get advised to also pay an immediate 10% to the state government, too. Some states (e.g., Alaska, Nevada) don't levy income tax.

If you don't pay income tax within 3 months of being liable for it (when it became your income), then you can be fined by the Fed or State governments.

Otherwise, working out American taxes is hideously complicated, because there are so many possible deductions from taxable income.  This is just a general guideline, for Fed tax only, for single people with no kids, using 2006 tax year brackets (it will change in the future).

You don't pay tax on the first $5000 worth of income.

You pay 10% for the next $7500 of income, 15% for the next $23k worth of income, 25% for the next $35k, up to a maximum of 35%.

So if you earned $800/week, that would suggest annual income=$41,600.

41,600-5000=36,600, taxable income.
7500*10%=$750
23000*20%=$4600
6100*25%=$1525.

Tax=
$750 + $4600 + $1525 = $6875.  That's 16.5% of the gross pay of $41,600.

Assuming you can't also deduct for mortage interest, childcare costs, capital losses, rental costs, private education costs, etc.