Things are sanitized when you expose them to a sanitizer, and the clock starts ticking immediately afterwards. However, the level of contamination over time can vary big time based on the environment. If you sanitize a bucket and leave it sitting outside without the lid on, I wouldn't trust it 10 minutes later. With the lid on? All day. Even when there's a chance that some wild microbes found their way on to your stirring spoon, there's no way to know if it's going to cause a problem. However, sanitizing right before use is cheap insurance.