You need to talk to a CPA. It depends on your level on income, deductions, etc. State taxes vary. Including those and SS, I pay about 50%. If you had SE income last year, you will also need to make quarterly estimated payments to avoid penaltites at the end of the year.
If you're making good money with adsense ($2,000+/mo), I'd definitely recommend talking to a good accountant. You can take advantage of a variety of adsense-internet related deductions to lower your taxes.