Tested it in Try Mode, seems to be about Five seconds per charge (10 seconds for max charges stored). Didn't use a stop-watch or anything but I'd say 5 felt about right.
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WAIT THIS WIKI IS dec 2014.
Adds a charge once every 5 seconds. Available to Arthas, Diablo, E.T.C, Kerrigan, Muradin, Murky, Sonya, Stitches, and Zeratul.
every FIVE seconds?! That sounds too short.