I'm assuming your neasighted (myopic), which means your eye is longer than it should be. As you get older, your eye will continue to grow longer and longer until you're in your early to mid 20s. Once you've reached this age, further ocular degeneration slows dramatically, and usually won't rear its ugly head until much later in life when you become at risk for corneas or faulty lenses'. Laser eye surgery corrects irregularities and smooths the surface of your cornea so that you're eye can properly focus the light onto the retina. While it's possible that you'd need to go in for surgery again in the future, it's unlikely your eyesight woudl degrade signifigantly unless you contracted a disease because your eye stops growing, and therefore getting worse, in your mid 20s.
Oh, and staring at a computer screen/tv/the paint dry on the wall for hours on end won't signifigantly speed up the rate of eyesight loss. It will, however, damage your eyes lenses' and muscles, because they're being strained by looking at one object for too long. Old people often suffer from shitty lenses that prevent them from looking at one object, then switching their gaze to another quickly because their lenses have become worn out.