One reason some people do not like the DX5 (and some other transmitters) is because it uses individual batteries in spring loaded connectors rather than a battery pack with a plugged in connector.

Futaba has timers on the 7C and higher models.
You can get a stand alone timer for a lot less than the price of a transmitter
http://www.talkingtimer.com/index.ph...products_id/30 This one talks to you for $15 plus shipping
Six bucks will get one that does not talk
http://www.amazon.com/Taylor-Digital...768566&sr=1-25

I get tired and start to lose concentration after about 10 minutes of flying. Some of the planes I fly will run for 20 minutes but I cannot do it for that long. My timer is set for 10 minutes and I land within a couple of minutes after it beeps.

At 3/4 throttle your combination will probably fly for a good while. One way to get an estimate of the upper limit is to secure the plane on the ground and time how long it takes to use a full tank at wide open throttle. Set your timer around that mark and you should be fairly safe since you say you fly at part throttle.