Return policy depends on the make and type of the monitor. A Pixel defect can be Bright pixel (White dot on Black screen), Dark pixel (Black dot on a White screen) or a Sub-pixel defect (visible as a coloured dot on a Black screen)
A Class 0 product should not have any pixel or sub-pixel defect, a Class 1 product could have up to 5 defects, Class 2 product could have up to 10 defects, and Class 3 product could have up to 50 defects. (These get further divided into the number of dead or stuck pixel counts but I mentioned only the total count)
Basically it depends on what you buy.. Try to get it changed.