I don't have a good gauge at this point, other than I know I haven't drained the batteries yet...
Peter Piconi (a fly shop owner and guide here in San Diego) was the one who introduced me to them; he uses them heavily when guiding on San Diego Bay in his much heavier glass boat, and I don't believe he's had any issues with batteries going out before his charter is done. As of this point, I've been using mine primarily to set myself up on a drift, occasionally to maintain position when I get into fish, and to quietly ease into shallower areas of the bay.
I'll be happy to post more info once I have more to share.
C
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Triumph 190 Bay
Yamaha 115/4, Lenco Troll 'n Tabs