Last summer I used the Whisper pretty much exclusively (I also have the Jam), and mostly went on two and three day trips. The total pack weight for a three days was right around 12 lbs., and that included a liter of water, camera, monocular, small book and other sundries that your average SUL hiker wouldn't be caught dead with. I was generally pleased with the performance, but be forewarned: it is DELICATE! While jamming a windshirt into the front pocket a fingernail sliced right through the fabric! This was easily repaired with Silnett. Now I'm extra careful to avoid branches while walking and never prop it up against a tree to guard against abrasions, just lay it down flat on some soft ground. As for total carring capacity, I'd peg it at about 20 lbs…maybe. Adding the optional waist belt would be highly advised if you're going to be carring that much. But to answer your question about maximum trip length, given about two lbs. of food/day, I'd guess about a week before resupply. For a 4 oz backpack, that's pretty durn good.
Happy Trails!