I'd say 150,000 miles. I think a lot depends on the car, how it's driven, and how it's taken care of, and could really be anywhere from 75,000 to 200,000 miles before it's really "high mileage".
Obviously a Civic given to a clueless 16 year old to grind the snot out of is going to have wear issues long before a Civic driven 80 highway miles a day by a commuter.