They do spawn every 8-9 hours I'd say, after you've clicked the first one. The problem is if you log in every 6 hours, you will miss the second spawn and only activate that timer after 12 hours, which means you wont get a third one in the 6 hours that are left. Yesterday I logged in 6 in the morning, half past three in the afternoon and then 1 o'clock at night = three spawns.