Why can gliders fly for so long?
A glider and a paper airplane operate on the same principle: Exchanging whatever potential energy (altitude) they have for the kinetic energy (airspeed) required to keep air moving over the wings so they produce lift, giving a stable, controlled descent (glide).
In both cases the airfoil will not stall unless it exceeds its critical angle of attack which is independent of the airspeed. You can continue to fly (with the wing not stalled and producing lift) as long as you still have altitude to trade for that airspeed, and if you do it right you will eventually run out of both altitude and airspeed fairly close to the ground, where you can have a soft landing at a speed where the aircraft is not damaged. In the case of a manned glider hopefully this happens on a runway where you can get another tow plane to take you up again.



Both aircraft can extend their glide time by finding other sources of  lift - thermals, ridge lift, etc. - to carry the vehicle to a higher altitude, providing more potential energy.  Gliders are better at this for one major reason: They have a pilot who can keep the aircraft in an area of rising air longer to gain the most altitude possible.

A paper aircraft can similarly be made to fly for a longer period of time if given an adequate source of lift (equal to or exceeding its minimum sink rate). There's a rather famous video of the "infinity paper plane" which is a paper airplane circling over an electric stovetop, but I'll spoil it for you and tell you that one is a hoax (the general consensus is the plane was on a string).
This one, however, can be replicated fairly easily: The result is more similar to a helicopter in autorotation or an autogyro in a glide than a traditional fixed-wing glider, but it's a good demonstration of the principles involved.