Short answer - atleast two weeks before.
However, if you want to have fully completed brows before you go you'll need a minimum of 8 weeks. After the first session we have to wait a minimum of 6 weeks to do your touchup session. The touchup session should then be given atleast 2 weeks to pass the most critical point of healing. At two weeks healed it is safe to apply SPF to protect the brows but I still recommend wearing a hat/sunglasses to further protect the longevity and quality of your permanent brows.