It is always better to apply floor coatings in the morning, this ensures that they have the warmest temperatures to begin curing. It is very important to ensure that the temperature of the substrate is above 5 degrees C. Below this temperature the epoxy will not begin to activate. 5 degrees C air temperature does necessarily translate to the same in floor temperature.
The sealer will usually be ready for topcoating after 24 hours.
However, it is important to test the sealer before topcoating to ensure it is fully dry. If the sealer is still soft or sticky, then a topcoat could entrap solvent in the sealer.