You probably want about an inch to an inch and a half or so of thread to go into the frame. A bit more won’t hurt, though you’ll have to check the thread length on the bolt. Take whatever the thickness stackup of the bumper, winch plate, and any other accessories (bull bar, hoop, tow hooks, etc) is, add an inch, round up to the next size (if there is enough threaded length) and order that.
Consider ordering Grade 8 flange bolts if you can find them. Flange bolts do a better job of distributing the load on the attachment. McMaster-Carr sells Grade 8 flange bolts
