How do you tell when it is time to water your plan...

She looks great. If she's that big in 2 weeks she's going to be a big one. It's very easy to over water in the beginning and a common issue especially when you have a small plant in a big pot. Keeping the humidity as high as possible 65-70% for seedlings is great until their root system gets bigger. Best way is to let the top inch dry to about the depth of your first knuckle. Next pick up your pot and feel the weight before and after watering it takes time to get the feel. In the first few weeks you don't need to water the entire pot. At the stage your at if you water a circular area about 6 to 8 inches around the plant you should be good. The water will moisten an area larger than you watered. Remember the root system is still very small. Lastly get your hands on an electronic moisture meter to help you determine how much moisture there is at different levels in your pot. I use one myself especially between week 3-6 after that they drink like crazy. Good luck I will follow your grow.