I remember this one. The two Fae Lights are spaced a little under 6 minutes apart. Of course, you're not going to see a second Fae Light now and you probably wouldn't need to invuln it either. That being said, the more useful invuln use for this fight was Superbollide on Thunder Rune (spaced a little under 7 minutes apart), and I'm fairly certain this also stopped being the case when Holmgang was buffed in 5.1 to have an 8 second duration instead of 6.