If you let a Beagle be outside for a full day, or live outside meaning sleeping there as well, a huge factor in the dog’s safety would be the weather; both heat and cold extremes.
Precipitation plays a role as well, since rain or snow would affect the speed at which hypothermia would develop.
In general, most of today’s canine pets (this excludes sled dogs, etc.) can easily tolerate temperatures between 50 F and 80 F without issues. Factors that affect a dog’s tolerance level include age, health status, and level of hydration, among others.
During cold weather, both hypothermia (abnormally low body temperature, below 98 F body temperature for canines) and frostbite are concerns for dogs that are left outside.
With hypothermia, the exact time that it takes for this to develop in dogs is dependent on the air temperature, wind chill factor, if the dog is wet (from rain or snow), and the dog’s age and health status.