There is no such hard and fast rule that such syndrome will occur after 2 hours duration
The best way i give you to prevent that anyway
1) Keep blinking alongside
2) Something which can take off your attention from your monitor can be helpful... like a TV a bit far away or a baby too..lol
3) Zip the Contrast of screen down to the minimum ...in which u can read atleast.
Screen may look non-lively...but Worth it
4) Zip the brightness too as much as you can
5) Artificial Eye-Drops can be used once a day esp during midway session while sitting...without any worries.
In last six yrs of heavy "PC usage"...i felt its sypmtoms just only for 1ce after using the above measures.
Hope that helps