Is it different to add seasoning at the beginning, in the middle or at the end of cooking

The problem is, both Sarge's and Carmi's answers are right depending on who is using the word. Some people use the word seasoning to just mean salt, some mean salt and pepper, and some mean "anything you want to use to bring the flavor to the desired point, including salt, pepper, lemon juice, spices, herbs, etc." If you are dealing with someone in person, it is best to ask. With a cookbook, sometimes you can figure it out from context or from other recipes. Otherwise, you'll have to make your own best call. In my own recipes, I will generally write "taste and adjust seasoning, adding salt and/or more lemon juice as needed".

The one problem I see with the article is that they cook the steak "as is" after letting it rest. They even mention that because so much liquid had been pulled out it caused problems with cooking. The missing step is to pat the steak dry. It needs to be dry when going in to the pan. I think the results are slightly skewed because of this - it was dry at the 0 minute mark and at the 40 minute mark but not at the in between marks. Keep in mind that you can wipe off seasonings when patting dry so this is still something to keep in mind.

As for your question - how long to leave it out? As Aaronaught said it doesn't really matter if it's salted or not. How long a steak can sit on the counter and not go bad will depend on the conditions and is more of a food-safety question unrelated to seasoning. You can, as the article mentions, place it in the fridge if you want to let it rest for a long time. Just remember to let it come back to room temperature before cooking for best results.

I'm not sure I really buy in to the importance of water reabsorption in how it relates to taste. The entire process of dry aging is designed to pull moisture out of the meat to give it a more concentrated beef flavor. Again, the article mentions that letting it rest this way for up to a day only results in an ~5% loss by weight, which pales in comparison to the 20%+ loss from cooking.

Now if you want to let it "rest" for a LONG time, like days or weeks you're talking more about a process like dry aging, in which case you'll want to start with a large piece of meat as the outside will need to be cut off as it will get rather dried out and not very appealing to eat. This involves more than just putting meat in the fridge though, so you probably don't want to go too far past 24 hours in the fridge.