I fly a 70" profile with a dle20. I use one 3000mah 2 cell Life to run it all. After 4 flights I have about 50% battery life left. I do run two switches. The battery pack has 2 servo leads so I just run one to each switch. There are many many variables. So if you go this route be sure and check you battery life every flight till you get a feel for the life of the charge.
I don't run a electronic kill switch. It's not a bad idea. I too have flown a plane for 20 minutes waiting for the fuel to run out. I would say there is not a lot of room in a profile for extra parts, but only one of my other 4 gassers has a kill switch. If I flew at events or contest it would be different.
David