Can I run 90 feet of 6/3 NM-B, carrying 220 Volts/50 Amperes, in schedule 80 PVC?
Your voltage drop is less than 4V so that is good. With 4 90's it would be legal to do it without junction boxes(360 max - then boxes are required). Soap the wire when pulling it will go much easier. We used to use Dawn dish soap but they also make "pull soap" that wont dry out and get sticky like Dawn.