Most sunscreens are formulated to keep their original strength for two to three years, if stored at room temperature. That means that you can normally use leftover sunscreen from one year to the next.
If however they have been exposed to higher temperature (which naturally often happens on sunny holidays!), they might expire quicker.
Also, you never know how long they have been standing on that drugstore shelf, so personally I tend to use ‘fresh’ ones each holiday.
Also keep in mind, that you should use sunscreen generously and frequently, so a bottle of sunscreen shouldn’t last longer than one holiday anyway. If it does, chances are you are not using it correctly. Remember – you should apply an entire shot glass of sunscreen to cover your body (and repeat every two hours)!
In any case, you always discard products that show obvious changes in colour, smell or consistency.