How low does your glucose have to be in order to eat a snack and how many carbs can the snack total?

15-20gm. If your bs is over 200, then it's better if you opt for a carb free snack, however at bedtime you should have carbs regardless of bs. Snacks should be in 15-20 gram range unless strenuous physical activity is taking place in which double the amount is advised.
Depends. People gets hypoglycemic symptoms at different level. You can eat a snack when you feel low (fast heartbeat, sweatings, shaking, confusion...) or if your bs is < 70 mg/dl. Follow the rule of 15. Eat 15 gm of carbohydrates (4 oz juice, 6 oz soda, 8 oz milk, 3 glucose tabs, 6-8 lifesavers) then wait 15 minutes before rechecking. If still low, do it again.