good question Vette,
to answer your original question
about how long would it take to have a block problem... just depends, if you had no antifreeze it would take about 3-4 days at 25 F and at -25 it would take about 30 min...says the expert
some say a 50/50 mix should be good for -15.... so as long as you have some antifreeze in it I would not worry about it..