As I have noted in a couple of other answers, I have worked in the CPG (Consumer Packaged Goods) industry for almost 30 years. It is safe to say that there is not a simple answer to this question. There are so many variables that it would be difficult to even go into all of them. Before I go further, let me say that common sense goes a long way.
Most food dating falls into three areas: Sell by, best if used by, and an actual expiration date. The one asked about, best if used by, probably has more variables than the others and is much more subjective.
First, it depends on the type of product. Is it a dry product, canned/bottled/jarred, or perishable?
Here is a little about each:
Dry products such as cereals, grains, crackers - These products vary in how long they will remain good. Keeping them well sealed and stored in a dark, cool environment will extend the time in which they would be considered good. Items such as cereals and crackers tend to get stale and unappetizing in a relatively short time. With those, common sense prevails and if it doesn't have the right taste or texture you naturally wouldn't eat it.
Can, jar, and bottle items - Items canned in actual cans can remain good after the best if used by date. However if you see a bulge, leak, etc. the item should be discarded regardless of the date. Food in dented cans should be looked at closely as the dent can cause a break in the seal. Other items such as bottled salad dressing will remain good after the best by date if they are unopened. (Given the choice of using an unopened bottle past the best by date and one that has been open in the fridge for 6 months, I would go with the unopened bottle.) For items canned in jars, look at how the food appears, be sure there is a good seal, and smell.
Perishable items - Storage conditions are extremely important. Be sure that your refrigerator and freezer are operating at optimum temperatures for keeping items fresh longer.
With most dairy products, if stored properly, common sense prevails. If it's molded, sour, etc. it's not going to look, smell, or taste good. An exception would be an aged, hard cheese, such as cheddar or parmigiana. You can cut past the mold and use the cheese safely. Many everyday dairy products can remain good well after their best by date.
Items dated in-store - Many items are shipped to stores frozen such as heat & eat items found in the deli bunker, branded meat items found in the regular meat case, etc. In this case, you want to look for the longest dated items. This is because the items come in frozen. When ready to place in a case, the store will take out of the freezer, date it per the manufacturer's criteria, and place in the case. Dating varies depending on the mfr. It may be one week, two, three, or even longer. (I have seen dating out up to 35 days on a heat & eat item. If most consumers saw that item in the case and the best by date was a week away, they would assume that it was pretty fresh. When I look at that item I know it has been thawed and sitting there for 4 weeks already!)
Most of the time "best if used by" dates are of a courtesy or guideline nature rather than a legality. The above information is only a slight overview and demonstrates how subjective this is. In closing, here are a few thing to keep in mind:
- Use your head. Common sense goes a long way.
- Remember that all food will deteriorate to an unsafe or undesirable point given enough time.
- Once product packaging has been opened, deterioration occurs more quickly.
- For various reasons, food can spoil before the best by date.
- Proper storage is important for all foods, not only for safety, but for the best quality and flavor.
- It's very important to be diligent with all canned items, regardless of whether in a can or jar. Dangerous toxins can develop which can't be seen, smelled, or tasted.