It depends on the kind of lifestyle you want to live, I guess. If you want to live in a $30M mansion estate with 24/7 domestic staff and have several vacation homes around the world, $50M might not cut it, while $250M should cover it just fine.
If you have a philanthropic bent and want to be able to fund various charities to the tune of $5M or $10M per year (in addition to a reasonably luxurious lifestyle, though considerably less than the above hypothetical), $50M might not be enough to sustain that over time, but $250M probably will.
I think it's fair to say that there's not much difference between net worths of $25M and $50M, or between $50M and $75M. But jumping an order of magnitude from $50M to $250M will let you live a very different kind of life, if you so choose.