There is a vertical cliff face in Snowdonia where with the correct wind speed (20kt+) and wind direction (up the valley) it is not possible to hover (at least in a Wessex or Seaking) for more than about a minute.
As you stop along side the cliff face in the vertical updraughting air you can hover on around half power for a few seconds before the lever has to be gently raised to maintain position. If you hold position you will start to run out of power and must fly out. Happens every time.