Fridge life of sauce/condiments Right so, for many sauces i buy, they have a use within 3 days / 4 weeks after opening in the instructions on the back. 

For most.. they have an expiry date on the actual bottle and "Refrigerate after opening" on the instructions, with no info on how long i can use it after opening, does this mean, if constantly refrigerated they will last until the expiry date (Which can be well over a year ahead) Despite having opened them a while ago? 

Examples : Teriyaki sauce, Honey/Mint sauce, Apple Sauce, Ketchup, Barbecue sauce, sweet/sour sauce, and the list goes on..
For starters, it is not the expriy date. For anything perishable, it is only relevant for the sealed bottle (and that not only for condiments) and does not have a meaning after you open it. If it is nonperishable, it is simply a best before date and irrelevant for safety. 

So there is no single answer to your question. Some will be highly perishable and fall under the 3-5 days rule for cooked foods, others will keep forever, and some will be in the middle. You have to look everything up in a shelf life database like StillTasty.