I thru-hiked the AT in 1999. While gear has changed, and information has become even more accessible due to the internet, may elements remain much the same. So I'll offer a few thoughts, based on that experience, and with modifications based on many miles on other long trails in the last few years.
First, you asked about average time. I've known those who finished the hike in under 100 days and those who took 8 or 9 months, but the average time is probably a little over 5 months. I took 171 days, including a 2-week break when I flew home for a friend's wedding. I was about 5 1/2 months.
Next, you don't need much in the way of maildrops. I would actively discouraged using them as your main food resupply. After the first month, every hiker I knew was sick of the same stuff and hiker boxes began to fill with rice and Lipton dinners.
Maildrops are useful for sending yourself the next several maps, or special items like disposable contact lenses or slide film or prescription meds and the like. For the rest, they are a pain. You WON'T save money by buying in bulk because you pay a fortune in postage, will carry more food than you need, and won't eat a lot of your food because you'll be sick of it. Buy from local businesses. You'll get better overal variety, save a ton of planning time, and get the chance to resupply out of hiker boxes at any number of hostels from those who DID do a ton of maildrops.
One option to help you around long stretches without good resupply options but with a post office is create a maildrop during a town stop. Then ship it ahead to the desired post office. By mailing it, say, 100 miles from the destination instead of home, you'll save a lot in postage and have fresher food.
Third, go as light as possible, but don't get too worked up over going with an ultralight or SUL load like so many here espouse. On a thru-hike you tend to carry a bit extra out of necessity. You'll wind up with some extra batteries, extra ibuprofen, a wallet, more food, and so forth that most dedicated UL'ers will consider excessive. Don't get too worked up if your base isn't below 10-pounds.
That said, start light. If your empty pack is more than about 3 pounds, you're probably going too heavy. This is why ULA and Six Moon Designs, two fantastic gear companies, sell their gear at Mountain Crossings Outfitter, 30 miles up the trail at Neel's Gap. So many folks are astounded when they see how light their gear can be, especially their pack and shelter. They've never heard of these companies because they are niche operations that predominantly sell online, but have a slam dunk market for thru-hikers with 30-38 miles of too-heavy gear experience. Look at these guys and other online niche companies now, not 4 days up the trail.
Last, join the Appalachian Trail Conference (ATC). You'll more than earn back the price with your discount when you buy maps. Also buy the AT Data Book (info to the nearest .1 miles and great for water sources) and the Appalachian Long-Distance Hikers' Association (ALDHA) Thru-Hiker's Companion. This book gives great information on town stops, services available, contact numbers, etc. It also gives some info on trail features, but is best for planning resupplies. It is MUCH more up-to-date and accurate than Dan Bruce's (Wingfoot's) Thru-hiker's Handbook, if he even intends to produce one for the coming year (it is rumored he is not, and it's no great loss.)
And have fun with the trail, but decide if fun is all you're out there for. If you only want fun, do sections and enjoy. If you want to thru-hike, accept that parts are NOT fun. Horrible trail conditions, foul weather (both cold rain and intense heat), and psychological burn-out are all a part of the thru-hiker's package. There are times when you'll get up and hike because it has become your "job". The cool part is when things look up and you realize you love your "job". But I recommend setting serious goals and benchmarks, and walking through the occasional disheartening times to that goal. My experience is that those who said "I'm only hiking as long as it's fun" didn't finish the trail.