Hello Marcus - you'll likely find a few different opinions on just how long your whisky will remain 'good'. It's not going to 'go bad' - it's just that the flavor may change over time if air gets to it. Over many years, air can seep into even the best sealed bottles causing evaporation and possibly altering the taste. Also, if your bottles are corked, then the cork itself may deteriorate over the years, causing the taste of the whisky to change. If a metal cap is used, it can also deteroriate over time causing a possible metallic taste. Keep in mind, these are just 'possibilities' - no one is saying this will surely happen.
The oldest open bottle I have is about 6 1/2 years old - and it's just as good to me as it was when it was first opened. It boils down to trial and error - keep your bottles as tightly sealed as possible and keep them cool, dark place which is considered ideal.