I voted 2 months, only because my newest styling gel hasn't been in the mix longer than that.
But it usually takes longer for me to decide it's an HG.
My HGs are:
Robert Craig's (RC's) Shampoo for Color-treated hair (HG for 9+ yrs, since I've been using his Color at least that long!)
RC's Conditioner (same: 9+ yrs)
CHS Curl Keeper
(about 2-3 yrs)
Suave Firm Control Gel (the newbie at about 2 months!) ***before the suave it was kenra's gel or mousse***
I get 2nd, 3rd and often 4th day hair with these products, just misting with water each day to refresh.