How long can an eye strain last?

straining the eye is just like your muscles getting sore from using them for a period of time or vigorously exercising them. If you have to use a computer for that long, just make sure you look away every five minutes to let your optical muscles relax by allowing them to be able to change focus on things that are at difference distances.

most eye strains last 3 days- 3 weeks, however some, if severe enough, can be permanent and may require medical attention.

I suggest a proper eye exam. Also some drug store reading glasses may help until then.