Is it bad to keep a solenoid activated?
Most common solenoids are driven by a short high voltage pulse (which is required to overcome BEMF and still provide enough current to keep the force). Then after the pulse voltage is reduced. Not to zero (this is release) but to some small value, so it keeps enough current~=force to hold it. And yes, if you keep the high voltage for long time- you will overheat and burn it, because basically it's just an inductor with low DC resistance.