1) Can't tell you if that's normal for YOUR laptop.
2) A good rule of thumb is that a 13" laptop with 50WHr battery will run about 10 hours in idle with 100nits, and 3 hours at 100%. Varies on model of course.
3) Batteries WILL discharge over time. Worse than that, charged batteries will deteriorate if left charged (or completely discharged) for extended periods. If you plan on keeping your laptop off for over a month, leave the battery at ~40% charge and put it in a sealed bag with moisture absorbent packs in a cool place (refrigerator works, but do NOT put it in the freezer)