Just energy consideration
One way to look at it is what is the potential energy of the aircraft; this is mass * gravity * height above ground, or mgh. In order to get a 500 Kg glider 3000 meters in the air, 500 kg * 9.8 m/s^2 * 3000 m ~ 15,000,000 = 15 Mega Joule has to be expended. With a glider that has 30:1 L/D, an extra 3% has to be spent overcoming drag. Assuming a motor glider, 15.45 Mega Joules, which is 15.45 Mega Watt Seconds. One horse power is 750 watts. So with a 45 Hp engine, climb to 3000 meters should be 8 minutes. (((500 * 3 000 * 10.3) / 750) / 60) / 45 = 7.62962963. But not all 45 Hp goes to forward thrust due to prop efficiency considerations, reality is more like 25 minutes.