What is the shelf life of a NiMH battery?

You probably mean to ask: What is the self discharge rate of a NiMH battery?  The rate of self discharge for any battery depends on the temperature at which it’s stored.  Stored at 70 degrees F (20 C) NiMH batteries will lose up to 40% of their charge within a month.  If they are stored at a higher temperature, they will self discharge at an even higher rate.  NiMH batteries stored at a lower temperature self discharge at a lower rate (Self discharge chart available soon).