I checked with a 'dog specialist and she told me to watch for 'paw lifting' like marching. It means their pads are frozen...another sign is if they stop playing and curl up in a ball to heat themselves...
In a crate, an 'airing' or keeping in garage, they will bark first to get in the house-)
hope this helps. So far i haven't seen them complain. I think i get cold first!!
I took alex out one morning (i dont know what the temperature was for sure) and it was COLD! I was waiting for Charlie (my yellow lab) to go to the bathroom and i heard a whimper. I turned around and alex was laying on the ground crying with her feet up in the air because it was too cold for her little paws... so i dont really know how long... good question