I wouldn't use a MH or HPS for more then 2 grows myself. They will last even up to 2 years if you wanted but the output would be greatly reduced, prolly would cost you more to run it and your plants would be getting less light.
As far as wattage vs..., with the eye hps bulbs the 400 watt says about 24,000 life hours, 600 watt 16,000 hours and the 1k watter 24,000.
As far as noticing it, some ppl can, some can't but your plants will for sure.