Assuming it's still 200W for Sentry (it's been months since I've seen any reports of measurements, and I never pay attention to usage since I rarely use it, but it's probably similar):
If you have 19" you can scale this number by 304/322 (0.82 rmi/hr)
So (for 18"):
14days*24hr/day*0.87rmi/hr = 292 rated miles over 2 weeks, 14 days.
So, assuming you have 322 rated miles at 100% still (may not be valid; that's why we don't do these calculations using %!!!), if you start at 90% (somehow), it'll turn off at 20% after about 11 days, and you'll probably have 15% battery charge left when you get back.
This is just an estimate, of course. But it's in the ballpark unless they somehow cut Sentry Mode to 100W or something (doubtful).