According to experts at Iowa State University, "If the material is applied too early, crabgrass seeds that germinate late in the season will not be controlled. If applied too late, some crabgrass seeds will have already germinated."

And according to Dr. Aaron Patton, Purdue Extension turfgrass specialist, “The preemergence herbicides (crabgrass preventers) that are applied in spring to prevent the emergence of crabgrass seedlings work only as long as they remain in the soil.” He said that generally, these products last in the soil about 60-120 days depending on a number of factors (rate applied, ingredient, turf cover, temperature, moisture, etc.).

So, applying crabgrass preventer as soon as frost has left the ground might or might not be too early.

Go here to determine when it's best to apply pre-emergent crabgrass preventer:

https://turf.umn.edu/news/spring-preemergent-applications-crabgrass

http://purdueturftips.blogspot.com/2015/03/common-questions-about-crabgrass.html