In general, I wouldn't say that ideas have a shelf-life. Ideas should be timeless. And although your ideas may affect people for centuries after you write them, your window of opportunity to write them may be shorter than you think.
My list of blog post ideas has entries that date back to the first month I started Cheff Salad. I will very likely never use those ideas. Even when I can't think of something and I peruse my list, those entries never seem to strike my fancy. It's like, when I wrote down the idea originally, I had every intention to use it, but because I kept putting it off, the idea has lost its energy and I've lost my interest in them.
I will admit that ideas can can come back to life, but I've found it quite rare. I will also say that some ideas live longer than others. But eventually, ideas will go away. Recording the thoughts will not be enough. If you want something to be written, get it done before the shelf life on your ideas expires.