The first time, it took 2 weeks or so, I sucked back then. Maybe it took more, I just remember me being proud to reach the last leval, but it took me a couple of days (if not more) to beat Devan. I didn't know how to cheat back then anyways. After that, I play SP rarely, when Im bored, I beleive I passed it without cheating in some hours, but its hard not to cheat for me (I usually do it to go back and get some secrets I missed to get a good end score). When I get bored or dont have enough time/patience, I just save the game and continue it later. Simple.