If we define samples manually how could there be a Sampling Frequency?
Units can be arbitrary, such as ISO Meters, or the length of the current king's foot (maybe longer than last year after the assassination of some shorter king).

A good default unit if there isn't some ISO standard, useful natural unit, or you can't think of one to make up (how long is your monitor's display, or your left foot, etc.), is the length of your window of data.  e.g. 1 cycle per your vector of 256, 2 cycles, 3 cycles, etc. per your vector length.  If your vector displays as 3 inches long on your display, then you can also easily convert cycles/vec into cycles/inch, or per/mm, per/fortnight, etc.