Last summer I was landing at Blackbushe and there was a helicopter (Dauphin sized) hovering off to the side of the runway not quite level with the threshold. I saw it and thought that it was far enough away that the wash wouldn't really be an issue. As I was crossing the threshold just before flaring the rotor wash pulled me straight towards the ground from about 10' up, I reacted quick enough not to cause any damage but had the stall warning going throughout.
Afterwards I went and looked up rotor down wash patterns and strength on the internet and found some interesting articles, which agree with M609. The outcome is, it does travel downwards and out up to I think it said about 10 times the rotor diameter, with it being stongest in the hover. They also tend to hang around longer
I would apply the usual rules for avoidance if possible or delay departure/arrival for a suitable time.
I've also experienced wake turbulence, the rotor wash was significantly stronger
I am now very aware of helicopters operating anywhere near me.