its going to be between 1 and 2 miles per hour lost, while on sentry mode. Keep in mind that is "battery use" because our cars dont store miles, they store energy, and that energy is used for everything from moving the car, ac, etc.
If you did this every night, 15 miles a night for you would be 105 miles a week, or, an "extra" 5,475 "miles" on your battery every year. If you drive 10k miles a year, your battery in this situation would be doing 15.5k miles a year. That may matter to you, it may not, either is totally fine.
The main thing would be for people to not use sentry mode when parked in a parking lot, and going on a trip (which is exactly when almost everyone WANTS to use it) unless that parking lot also has them plugged in. the car will lose between 20-30 ish milesevery 24 hour period in that circumstance, so a 7 day trip with sentry mode on the entire time while someone is away will use 140 to 210 "miles" just sitting there, at least until the battery gets to 20% charge.
That extra energy has to be put back somewhere too, so if you drive 10k miles a year, but used sentry mode and that was an extra 5k miles, you just increased your charging cost for the car by almost 1/2 of your regular usage.