Ask the hatchery where you obtained the eggs to determine the number of days to egg hatch at a particular temperature. This number can be different for different species and strains of salmonids.
A standard rule is that eggs of Brook Trout will hatch in 44-50 days at 50 degrees F. Or, for other temperatures you can calculate that hatching will occur when 900 temperature units have accumulated. But, remember to count the days the eggs have been at the hatchery and the hatchery water temperature, when applying this rule.