I have been making cookies for decades and I have noticed a dramatic change in quality based on the usage of vanilla. Using weak vanilla or just not enough will lead to bland cookies, oftentimes regardless of the usage of other ingredients. This has lead me to add twice to three times the amount of vanilla that is called for in my recipes. So far, I have been happy with the results. Is this a common experience? Is there a recognizable upper limit to the addition on vanilla? Or is it that, since I am a fan of vanilla, I am simply making cookies to my personal taste?
How much is too much is a totally subjective thing. Yes, there is such a thing as too much, but where that point is will vary among tasters. There is no objective question here to answer. No one answer to the question will be "correct".
I really like vanilla, and I like flavors to be on the rich side, so I tend to be a bit heavy-handed with vanilla. A few (very few) times I have been surprised to have that heavy-handedness criticized. Why would they not like my amazing vanilla ice cream? They experience vanilla differently than I do. What is ideal for me may be overdone, even cloying, to my neighbor.
So, when it comes to various flavorings (vanilla is just one of many), get to know your own "likes" and what your audience likes. After that, you can ignore what the recipe says and just do what feels right to you, knowing your audience and the potency of your ingredients as well.
Even if the price of vanilla has dropped significantly over the past years (at least in Europe), vanilla is still considered a quite expensive flavour. Perhaps that is also why recipes often operate with smaller amounts of vanilla than what is felt to be right.
Just as with any other spice or natural flavouring, it is also very difficult to determine a fixed amount for a recipe. Depending on the quality, one vanilla pod or one teaspoon of vanilla extract can be within a wide range of actual amount of flavour.