You can't return for 90 days. You're allowed 90 days without a visa in a 180 day time period. Also beware, There's a long list of countries that belong to the Schengen Agreement. They share the 90 days. So if I stay my full 90 days in Spain, for example, I've used up my time in France as well. Americans can stay in Mexico, England, and Canada for six months without a visa, however.