I have been doing a lot of testing with the smartphone app. I did an overnight sail on a 40' Trimaran from Jacksonville to Daytona and stayed about 3-4 miles offshore. I had decent cellphone coverage the whole time and did not see any times where it was not transmitting.

I had both a SPOT and smartphone running at the same time. Below is the link:
http://kws.kattack.com/GEPlayer/GMPosDisplay.aspx?FeedID=1388

During the Steeplechase last year, all boats were tracking without gaps in updating and so far, I have not had anyone reporting excessive battery consumption. Other than me when I was tracking, checking the track online and navigating with my phone all at the same time.