Without painting, the door will last for a while; with a good coat of paint it will last longer.
Early on when starting this project I looked at a door from a steel siding company that was all steel framed. Problem was it was like 400 bucks for a plain jane one. I paid 210 for this 9 window door. Most houses would have a storm door mounted to the brick molding (which is a major cheap product too) and that would protect the framing. I removed a door from my house 25 years ago and the hole framing was cedar (house built in 1926). The one I replaced was the cheap pine but it hasn't given me any problems because it's in a sunroom porch away from weather.I took an option that makes bottom 8" or so of jamb of plastic.