It can take up to a month or longer for grass to green up after being fertilized, depending on the type of fertilizer used and the weather conditions. If you've used a slow-release fertilizer, it could take up to six weeks for the nutrients to be released into the soil and absorbed by the grass roots. In cooler temperatures, grass growth is slower so it may take longer for newly fertilized grass to show signs of greening up. Warmer weather and more sunlight will speed up the process. Be patient and give your lawn time to absorb the nutrients before mowing or applying any additional chemicals.