I have re-nickelated Ni-NTA-agarose resins with varying success. I
believe Qiagen provides a protocol for doing this. If I remember
correctly, it involves stripping the resin with EDTA and then passing
nickel sulfate over the column.
Alternatively, I have also used Ni-NTA columns many more than 5 times
and have had pretty good resolution. Since these resins are available for
about $5/ml, I would suggest using the resin as long as you still get good
resolution and then either replace it (if it is a relatively small
volume), or attempting to regenerate it if it is a large volume.