How long depends on a number of variables for any engine, test stand or airframe for ground running. You need to get yourself a remote sensing thermometer and run it a little bit, check the temps and do it again. Then you'll have a better idea if there is going to be a problem with heat and how long you can ground run one before it starts getting too warm. You don't want to overheat the engine, any size engine, on the ground or in the air...