Heres my answer...
If PS4 had Backwards Compatibility, I would play PS3 games all the time. Then my answer would most likely be "Everyday". But right now it's a bit of a hassle to use. My surround sound is connected directly into my PS4 Pro via optical, my tv box is connected to my surround sound by HDMI. My PS3 is connected to a Samsung Bluetooth Soundbar. So when I use my PS3 , I hate the Soundbar sound, it's not as great as 5.1 obviously.. so yeah... right now, I use it maybe once a month. But think about using it everyday and lament the fact that PS4 doesn't have backwards Compatibility :(