The shelf life of a recipe kit varies based on what type of ingredients come in the kit.
Yeast- Liquid yeast should be used within 3 months of the production date for best results but can be viable for up to 6 months but a yeast starter is recommended for yeast that old. Dry yeast can be viable for up to 1 year if stored at room temp and even longer if kept in the refrigerator.
Specialty Grains - Un-milled specialty grains can last for about a year. Milled grains will stale quite quickly and should be used within 2-4 weeks of crushing.
Hops - If the hops are packaged in foil and have been nitrogen flushed they can be stored for up to a year in the freezer, shorter if kept in the refrigerator. You can see a decrease in overall bitterness the longer they are stored.
Malt extract - Liquid malt extract can be stored for over a year and still be fine form brewing but oxidation of the malt will increase with time and can affect the flavor and aroma. Dry malt extract can be stored for 1-1.5 years as long as it is kept in a cool dry place and is not exposed to moisture without a significant decrease in quality.
To obtain the best results I would recommend brewing within 2-3 weeks of purchase for any kit, but you do have some flexibility if you need to put off brewing. This is assuming the ingredients you are buying are fresh.