Remember that people didn't have fridges until recently and, as such, used to keep milk in a larder cupboard - where admittedly it'd be reasonably cold - for a couple of days.
Yeah, depending on how close to expiry it was you can usually leave milk out of the fridge for a day or two. In fact I recall seeing a table somewhere which showed you how long you can leave milk uncooled in certain circumstances. Think it was on a milk carton, actually.
Generally the rule is nothing should be out of the fridge for more than 2 hours. Although alot of people dont follow this (eg sandwiches for lunch etc)
But as people say look at it (has it gone lumpy or changed texture) sniff it and then taste a bit. If it were me id just not drink it and get some more later.
I drink milk that I've left 18 hours out the fridge all the time, I think after about 24 hours you should think about throwing it away, this is either right or I'm just hard core Stomached😈😈😈😈😇😇😇😴😴😴😴
If you had a gallon of milk unopened but out for 2 hours and it's at 80 degrees, sorry milk isn't expensive enough for me to take a chance plus the quality has probably degraded significantly. Opened dairy products in a home setting I wouldn't leave out longer than 2 hours.