To be fair the investigation wasn’t over when Nixon resigned. He just got tons of pressure after doing exactly what Trump has threatened (firing special counsel and recording conversations). Key difference is that Nixon was actually pretty smart.
Anyway the rumors are the investigation should conclude near the end of this summer. I personally am very interested in the 5 people the special counsel is requesting immunity for to testify. Not as interested in the Russian chick since she was actually nabbed in a completely separate investigation.