Currently all EU tourist visas (automatically issued when you enter the country) are valid for 90 days.  You can automatically renew your tourist visa by exiting the country and re-entering.  There used to be a requirement to be out of the country at least 3 days, but that is no longer the case – even an hour or two suffices for the renewal, though you might have to stand your ground with some officials looking to extort you, particularly at the Nicaraguan border.  There are also rumors that you will soon be able to renew your tourist visa at a government office for $150 or so, instead of having to get to a border, but no official word yet.