Defining characteristics of Southern Rock:
*Characteristic blues-rock sound, rhythm and sensibilities
*Sound driven by electric guitars
*Strong reliance on guitar solos and jams
*Typically strong southern themes and imagery
*Detectable influence from earlier styles of music including roots rock, blues, folk, country and general Americana
*Bands are almost exclusively from the South.
When trying to define "Southern Rock" as a distinct and definable subgenre in and of itself, as opposed to just rock from the South (as so much early rock & roll was), I don't think we can go any earlier than the Allman Brothers in the late 60s without losing the definition entirely. For instance Creedence Clearwater Revival employed a lot of the elements later associated with Southern Rock before the Allmans, but they weren't from the South and aren't usually considered practitioners of the genre.
As for when it ended, it's a tricky question. The classic period of Southern Rock died out in the mid-80s, after which point new Southern Rock bands stopped emerging and the old guard stopped charting. However, not only is music from the traditional Southern Rock period still as popular as it ever was, if not more so, but many recent bands, in and out of the South, are perceptibly influenced by Southern Rock and Southern Rock bands.
