Garmin forerunner 310xt has a running time of 15 hrs 27 minutes. Don't ask me how I know, I just do.

i use the garmin 405, does exactley what it says on the tin. shoes distance, elevation, calories and you can keep a track of your workouts on your PC with a wireless dongle supplied.great watch and priced at around £120 its a good investment   

great watch (and perfect for a triathelete too) but is £260-300 ish so not cheap. garmin 305 is snip at sub £100 at the moment by comparison. debra, what sort of ultras are you planning? just over marathon like 33 miles or 40-50 mile ones.305 might scrap it with battery life of 11 hours possibly.  405 is more like 7-8 hours. surprised you haven't got something like a garmin if you have perhaps already done half and full marathons.foot pod type watches (including garmin) will last a lot longer but not as accurate, especially if/ as you change pace/ stride length. googled a few articles with people running with a 305 and little portable power pack for £10-ish. something you could stick in back pack maybe or nearby. I've got a bigger one of these packs for a phone so could see this working.405 doesn't have a usb port I don't think so not possible. or buy another GPS watch, borrow one of a friend and swap half way. really depends how far you're going.