Can a glue gun cause a fire after being on for 15 hours?

Depends on how much heat it put out. Or if it can overheat, which most glue guns can so I wouldn't risk it. If you really can't help but leave it on then after two hours the wood table would be hot to the touch. In six hours it would be scorched, black streaks at the point of the gun. also, any plastic within 0ne to two inches would be melted or melting. In eleven hours the table might possibly be on fire.
Now you have your answer... and your table is on fire.
Good luck!