How much time do I have left on my iPhone 4s
While I do not think there is any official testing data that can accurately answer your question, turning off your iPhone should more than certainly leave you with battery 6 hours later.

In the past, I've tested out the standby capabilities of both my iPhone 4 and iPhone 4S. My test consisted of charging the iPhones to 100%, switching them to Airplane Mode, and locking the screen. I left them in that state over night (about 9 hours) and checked the battery percentage. They both reported 98% to 100% capacity, which surprised me greatly.

So I would like to think that you powering your iPhone 4s for 6 hours should keep the bulk of your charge in place. Maybe left with 25% of charge.

Of course, this is all assumption ;).