I definately agree that the rate does seem a little fast, but if you look at the game clock (not the one on the windows bar at the bottom of you\'re screen) an hour in-game is more like 15 minutes out of game which helps a little.
perhaps how fast you heal should be based on your skills and so be different for every player.
generally I do agree that it should slow down, but then maybe magical healing should have a slightly greater affect, right now I can barely heal faster magically than naturally (but I am a low level healer so that may be that problem)
oh, and I love the idea about the sewer water slowing/reversing healing for a while, and that you should be able to heal all the way naturally. Kudos! very realistic.