In mild to moderate cold temperatures your furnace should run at least 10 to 15 minutes to bring the temperature back up to where it's supposed to be.
The furnace should cycle on an off and run 2 to 3 times per hour and if your furnace is cycling on and off more than that or takes longer to heat the home up then your furnace is likely too small for the home or the furnace is going bad.
Now if the temperature outside is really cold then the furnace may run 30 to 40 minutes every hour to keep the temperature in the home to the desired temperature.
Also a furnace should not run constantly and if it does then the furnace is either too small or something is wrong not allowing the furnace to shut off.
If the blower motor never shuts off then the temperature sensor for the blower motor could be stuck and need replaced.