Complex sentence: They spend the vast majority of their time sunning on rocks, even in the heat of the day, until the surface temperature reaches approximately 104 ° F, when they will retreat to a rock crevice or burrow.
Split into simpler sentences: They spend the vast majority of their time sunning on rocks, even in the heat of the day. Up until the surface temperature reaches approximately 104 ° F, when they will retreat to a rock crevice or burrow.