Yeah, they dry from the outside in and as the dry layer on the outside gets thicker the remainder takes longer to dry. That's the difference between "Dry To The Touch" and "Cured". Plastic models make things worse since no carrier vapors can escape through the plastic.
Just as a matter of possible interest, concrete cures the same way and it may take a year or longer for thick concrete to completely cure in the middle.
Acrylics dry and cure rather quickly so, as Mike said, 24 hours should be sufficient if the environment is not cold and damp.