Is it okay to travel from Belgium to USA without visa, and how long can I stay? I have belgium passport.?

Simple, check with your local travel agency
about this one.

- - - - -

yes, your okay to travel to USA for 90 days b/c...

The Visa Waiver Program (VWP) allows Belgian citizens (and citizens of designated countries: for a complete list click here) to apply for admission to the United States as visitors (traveling for holiday and/or business, in transit) without first obtaining a non-immigrant business/tourist visa at a U.S. Embassy or Consulate.

Yes. See below for an explaination of the visa waiver program

90 days