There are those who say that the substitutes degrade over time. There are also those who say that it doesn't. I don't use the stuff now but did several years ago. I can't say for sure whether or not it actually degrades but when I did use it, it seemed to me that after about a year, it began to loose power. So, based on my limited experience, I would say that if it does degrade, it will become somewhat weaker over time. That being the case, I'd say go ahead and use it so long as you are satisfied with its performance. I would expect it to only get weaker not hotter with time. If it doesn't perform to your satisfaction, it will make good fertilizer so just sprinkle it on the ground and forget it.