Ozz,
That is 15 minutes of inactivity and is a good setting. You should not loose anything when it comes back up. Everything you were working on should still be on the screen. I have a Toshiba laptop and that is the way "it" works for me. Computers are different. A long time ago I had a shut down that I could not easily explain. Then it dawned on me that the little CPU fan on the bottom was being blocked by my fat lap. I now use a food tray used in cafeterias, upside down, on my lap. The little rubber pads give it enough clearance to keep the fan unobstructed. An old piece of plywood would server the same purpose. Unless you leave it unattended a lot, I would leave it at 15 minuts - conserves the battery.
On the battery - it is better to let it run down and give you the notice "low battery," than to leave it plugged in all the time. Keeps the battery from getting a "short temper" memory.
I have had my laptop since 2005. Had to replace the power cord/supply once, but nothing else.
Rots of ruck!
Edited: Here is some information on
Power Management