Officially most monitor manufacturers follow the ISO 13406-2 class 2 standard, so roughly 2 dead per million pixels or 5 dead sub-pixels per million pixels.
Dell have a premium panel guarantee on their higher-end monitors (the ones with the 0 in the code, e.g. 2408 is premium panel, 248 isn't) which will replace with even one duff pixel.
Finally you could also take advantage of the Distance Selling Regulations if bought online and just have to pay for return postage (you do not have to pay a restocking fee or for outward delivery no matter what the retailer says - if they claim otherwise report them to the OFT and look at performing a CC charge-back). You will have a duty of care to look after the monitor and return it in an as-new condition however (with all manuals, cables etc.).