depends on the size of the screen, the threshold for proceedure is generally below 26" gets replaced quite easily instore, but over 26", if it's from the place I work, the rules are as follows:
if it breaks within 28 days you get an instant replacement, after that into the manufacurer's warranty, you call the service number, they try and diagnose it over the phone, if that can't be done, an engineer will come out to take a look, depending on if you took out the "extended warranty" they might take it away, if it can't be fixed within 21 days they give you a new one, failing that, if it's just the standard warranty, it'd be sent back to Samsung where they'd deal with it and i'm not sure on their repairs proceedures, but you are entitled to have it fixed for free, maybe not an on the spot replacement
hope this helps
PS DO NOT take the tv into the store, they can technically book it in for repair there, but it'll take much longer, and they'll probably tell you to send it off yourself anyway, so this will save you the hassle of packing it all up, taking it down there and being told to take it home again