hmmm, now that is concerning. I don't make many calls but due to the nature of my job...at bare minimum, I'd keep a messenger and push email running. I currently average to about 2 to 3 emails an hour, several hundred text messages and IM's a day, and I'd hope to add roughly a half to one hour a day of web usage (google maps, cigar related forums, gizmodo.com) and ideally a half hour or so of bluetooth tethering to a laptop.
15 hours would probably do it for me but it sounds like, if I understand your estimation, I may be pushing it.