You should always put sunscreen on. Both UVA/UVB rays are harmful. Most urge applying a 15 at minimum (and larger numbers really don't protect to a larger degree). Of course, I admit, when I'm getting a tan I'll use 15 my first few sessions. I will drop as low as a 4, but at that point you're not getting much if any protection. You should not spend hours tanning anyway. It's not a good idea. Just because you don't burn does not mean you are not getting unhealthy amounts of UVA/UVB exposure. Even if you're not concerned about the cancer risk (and you should be), then worry about the premature aging of the skin. Do you want to look all leather-faced in your 30s and 40s? Later, if you're really lucky!