Hi, welcome to GoY, do you currently have weeds growing on this land?, if not there's no point in applying glyphosate, as it is inactivated when it comes into contact with the soil, it's a systemic weedkiller, which means it's transported throughout the plant, mainly through contact with the leaves, it wont stop weeds from reappearing if you just water it onto the soil, no matter how much you put on, you would just be wasting your money, but it's your money, so the answer to your question is ; as soon as you like, Derek.

Additionally, that stuff will leach down into the waterways and aquifers and is detrimental to fish and wildlife.