Different for everyone.
If you are running SS it's less critical than Fixed because the downhills won't matter as much.
Easier to express in Gear inches: 60's is very casual 70's is casual still but fast, 80's is harder probably best for flats, 90+ mostly for track and show-offs.
So something likes 44-18 would be in the 60's, 44-16 would be in the 70's, etc etc...