When do tankless water heaters need to be descaled?
The manual will say something like


  Flushing the heat exchanger with a descaling solution if mineral build up is evident. Scale build up will shorten the life of the water heater, descale heat exchanger thoroughly and repeat annually depending on mineral content of ground water.


(From Bosch 2400E NG user manual)

or maybe something like 


  Periodic descaling may be necessary in areas with high
  mineral content in the water. Scale buildup in the heat
  exchanger may result in lower flow rates, error codes of
  A7 and E9 and boiling sounds (knocking and banging)
  in the heat exchanger.


(From Bosch GWH 2400 ES NG User Manual)

so


  Under what situations should a tankless hot water heater be descaled?


When mineral build up is evident on annual inspection.


  Does it depend upon how hard your water is?


Yes


  If descaling should be done periodically, how can I determine the right frequency for my situation?


By inspection.