The standard advice we all ignore is: Never hike alone. It seems that since backpacking is a more highly complex kind of hiking, the directive to never go solo should be even more of an imperative.
If this were true, however, there’d be precious few soloists schlepping overnight kits down trail. As Philip at Section Hiker notes, the logistics of taking several people camping for several days makes it borderline impossible to find anybody go backpack with, even if you wanted to. Going it alone is the option that remains because so many our pals prefer not to sleep on the ground.
In a lot of ways, day hiking is far more hazardous than backpacking. You take far fewer supplies, you pay less attention to details like footing because you’re not carrying a load, you take on more distance than you might be able to handle because you’re forced to come back to where you started, vs. just finding someplace to bed down for the night when you tire out.
Backpackers implicitly take survival supplies along and have at least a few days’ worth of calories and some vestige of shelter if they get too terribly lost. That’s a nice margin of safety.
The main limit on solo backpacking is that you really should leave a detailed route plan and itinerary with somebody back home, and then stick to it.
The real measure of soloing should be: how alone will you really be? Two weeks on the John Muir Trail in mid-August will seem like Highway 101 at rush hour compared to a weekend on the same trail in the middle of October. I read about one guy this summer who plotted an early JMT thru-hike and found himself alone in the High Sierra for days on end and decided to call off his hike because he didn’t feel comfortable out there all by himself.
Amy Racina, author of Angels in the Wilderness, fell down a cliff in a very remote section of the Sierra where almost nobody goes; it was a millimeter shy of a miracle that some hikers found her and she survived (note: her backpacking supplies kept her alive for several days till she was rescued).
As long as you won’t be truly alone, there’s not that much risk to solo backpacking, beyond getting sick of having only yourself for company. Which brings us to a nice question for the crowd: what do you do to avoid boredom on camp-outs in the downtime between, say, dinner and your ultimate sack-out time?