There are two ways in which a campaign can be run. Either, we slowly optimise it with minimum budget to prevent overspending or we start big and spend budget quickly to see if we are getting the desired results or not. Second method could lead to results quickly but they may not necessarily be in your favour. I use the first method, so the statements below are based on it.
If you refer to results as impressions, you start seeing them as soon as ads start running.
If you refer to results as clicks, you start seeing them within 24 hours unless we start with a low bid. In that case, you should see them in a few days. It's done to prevent overspending.