You can give the seedlings upto 24hrs of light in its first few weeks. But i just go by the normal sunlight hours for over here (england) which is average of 16hrs.

Direct hot sunlight will kill the plants or seriously dry out the soil and cause you to water every day or 2. But realistically you should move your plants to a window that has a bit of shade, so the light isnt FULLY on the whole plant. A good shadow source is the window panes on the windows. They act as good shade and will decrease the amount of sunlight on the pot by about 30% that usually does job for me

Keep us updated and (if u aint already) start a grow journal, would love to see how your papaya come along

See ya in a bit,
Sam