How critical is Dust inside laptop

Just watch your heat over the years. It's according to where you live as to how much dust build up you get. Also I've found that households that use a vacuum cleaner a lot are much more subject to dust build up in the machines. The dust stays in the air longer and your machine acts like a filter. Usually a machine is good for four years before you need to get in and clean out the heat sink vents and reapply thermal paste. But I've also seen up to 90% blockage of those vents. So, if it's not getting hot, don't fix it.