I got a Pebble a few weeks ago. I use it for sleep tracking as well as daily wear, so charging at night isn't an option. I've gotten into the habit of charging it every morning while I shower and shave. Even when I'm extra-quick with my morning routine, I still find that the 15-20 minutes is enough to bring it back to 100%.
From everything I've read, lithium batteries actually do better when they're not discharged all the way—the only downside being that the device's battery gauge can lose calibration over time (hence the recommendation to do a deep discharge every 30-50 cycles or so).
This habit also will extend the useful life of the watch for me: Since I'm already in the habit of topping off daily, when the battery begins to lose capacity (down to 3 days from the initial 7, for instance), I won't even notice.