First Ever Grow

You probably would't need the second one until early-mid flower. And there is an advantage to keeping it: assuming you have adequate soil/space, the limit on your grow is your lighting. The rule of thumb for a grow is that a beginner should expect, if no catastrophes, about 50% of the wattage pulled from the wall of the grow lights. Assuming each draws 100W, then using one means 50 grams would be a reasonable expectation for your first grow. Two of them would double that: you could expect 100 grams from your first grow. And using the "one gram per watt (from the wall) rule, 200W from two lights means you could aim for 200 grams with 2 lights whereas 100 grams would be the ceiling with 1. Either way, the big thing to pay attention to is keeping the light in the sweet spot. When its there, the leaves seem to "pray." Too close and they'll curl, then bleach, then burn; too far away and photosynthesis will slow to the point the plant can "stall." Though that's part of the learning curve and part of getting used to your lights.

Hello I think 3 weeks of veg in a solo cup is about perfect. Some people will go longer and especially with a photoperiod strain this isn't a problem. However the more built up the roots get in there, the less happy they'll be and more likely to stress out. So I'd say go ahead and transplant into the final container. Personally I like bigger pots...bigger pots, usually bigger plants and bigger yields. Happy growing. ✌️

Unfortunately with a auto u have no control over when it starts to flower. That being said 3 weeks of veg is on the low side of the spectrum. Gauging in what I've seen of that variety, she will almost double in size in the first 2 weeks of flower. Meaning she will get bigger but ur looking at about a 12 inch plant or 1 foot. Happy growing and stay lit fam.