It all depends on their size and activity level. Size is the biggest drive. My guy was 10in and did fine in a 75g. He was extremely active and used all of the space which is not typical. I wish I had given him the 180g earlier as he always loved to swim all over that tank. However, many rhoms just sit in a corner.
I think a 75g starts getting small once the rhom gets around 12in. I would want to keep them in minimum of a 120g (4x2) or 180 (6x2), however, I am sure lots of folks keep rhoms in a 75g for life.