When to consider new decking vs full new build?

There is not set answer as the condition of every individual deck can vary; however, for general planning purposes if your deck is less than 15 years old and was built correctly you might be able to save it by replacing the decking and doing some minor framing work. If it is 20 years or older than you probably need to seriously consider new framing as you would not want to put new decking on a frame that is not going to last.