The solar cycle is reaching its peak and the observation of solar gamma-ray flares is now possible with the Fermi Large Area Telescope (LAT) which was launched 4 years ago. With its unprecedented high sensitivity, the Fermi LAT has already observed high energy gamma-ray emission (>100MeV) in more than a dozen flares. This presentation will focus on long duration solar gamma-ray flares above 100 MeV, which had only been observed a handful of times in the EGRET era for extremely bright X-class flares. Surprising discoveries were made by the initial Fermi LAT observations of a modest M3.7 flare on 2011 March 7 that showed sustained gamma-ray emission up to 14 hours after the impulsive X-ray flare. A preliminary systematic analysis of other major long duration solar flares detected in the LAT will also be presented.