I am here on a E2 Visa but I plan on quitting my job. How long can I stay in the US before I must leave?

Is there a grace period (i.e. 90 days) before I must leave the country? I do not plan on returning to my home country in Europe - and plan on marrying a U.S. citizen (we've been together for a year now) within the next year. Are there any options of getting any kind of visa...and is it really legit if I travel to canada or mexico every 90 days and come back on a visitor pport?