After a few hours surfing the web for reputable sources, I CONCUR with Brian Waspi. Its the amount of free chlorine in the water and how much of it reacts (= time * reaction speed) is what determines how many nasties will be chemically busted-up.
Determining the quantity to use is difficult given amount of NaClO in water ratios vary across bleach brands, age and sun decrease the amounts, and assorted things about the water affect the effective chlorine level. For example, to treat the toughest nasties in water, Portable Aqua Pure* (salt + water + electricity = bleach) include chlorine test strips to insure you've enough chlorine (and you still need to wait ~3 hours)
Having used bleach as a kids decades ago, there's also a taste issues. Some plain bleaches taste horrid so I stuck w/ plain Clorox.
Another reason to carry bleach is to sanitize dishes and such in larger groups. With Boy Scouts, we use a capful of bleach to sanitize dishes after the wash and rinse cycles.
I own and teach Scouts about a dozen different water treatment methods, but I carry a gravity filter, Aqua Mira drops, and a few tablets in foil packets.
*http://www.potableaqua.com/products/pa-pure-electrolytic-water-purifier/ Btw, the device's manual + miox.com is pretty good on the chemistry aspect