The rule of thumb I have seen in several places is no lower than 62F. If you drop 10 degrees and it takes all day for you system to make the difference back, there are likely other problems like insulation or undersized unit. It is a fact of life that, if your furnace was installed by the builder, it is the dead minimum that will meet code and likely will not meet your comfort needs. A lot of frozen pipes are crappy design or build issue that are just masked over by dumping more heat at the problem.
