Mathematically I think they should keep increasing, but I've only boosted the health with one character to 500+, and I'd need test the rate to be certain, tricky to get any accuracy on the 360.
And with enchants I've seen my health bar fill back up rather nicely, but I don't have any formal number to support my observations yet.
But at any rate the UESP wiki states that Health "Regenerates 0.70% of your maximum Health each second both inside and outside combat." http://www.uesp.net/wiki/Skyrim:Attributes