A Stones fan rocks out on vacation quite frequently (2,5,2,3)
I believe the answer is:
as often as not
'quite frequently' is the definition.
(I've seen this before)
'a stones fan rocks out on vacation' is the wordplay.
'rocks' indicates anagramming the letters.
'on vacation' says to hollow out the word (remove centre letters).
'out' with its middle taken out is 'ot'.
'stones'+'fan'='stonesfan'
'stonesfan' with letters rearranged gives 'softenasn'.
'a'+'softenasn'+'ot'='AS OFTEN AS NOT'
(Other definitions for as often as not that I've seen before include "Quite frequently" , "in about half of the instances" , "At least half the time" , "50% of the time" .)