They do not put "expiration" dates on food. They put "Best before" dates.
Meaning that AFTER that date, the food quality starts to deteriorate. And in some cases, the food may start to grow bacteria.
With whole grain products (such as wheat germ, or shredded wheat, etc), the oils in the product CAN start to go rancid after the best-before date. And that can mean that if you eat that food, it will upset your stomach and you will vomit it back up .. and then be fine.
There is not set time frame for "how long". It depends on kind of food, on how it has been stored. Dairy products and meats are the most likely to potentially make you very very sick after their best-before date. Whole grains not as likely.