I was thinking about this a couple of days ago, I won't say where I was at or where I was doing, but I was thinking not only what our idea is but what a readers idea of what a "First Impression" might be. In the Nintendo eShop, you can't submit a review (albeit a pretty short review that's limited to if you think the game is casual/intense, if it's for everyone/gamers and the number of stars out of five you would award it) until you've played the game for at least an hour. Granted if you hate a game after the first five minutes, you could just leave the game on for the next 55 minutes then give it a scathing review.
For me, it really depends on the game. For Cogs, after about 10 minutes, I knew that it would be a game that I wouldn't either be able to or want to finish due to my pseudo-dislike and frustration with sliding puzzles. Presently I've put in 56 minutes in Cogs and I probably won't play it again unless we have people over who happen to really like sliding puzzles. After those 56 minutes though, I felt like I could write a First Impression about Cogs (which I did) and feel that I had a good feel for the general purpose of the game. At the moment, I'm currently playing The Elder Scrolls III: Morrowind and 27 hours in, I feel like I'm almost at the point to where I could write a First Impression.
So why the huge disparity from one game to another?
I'll give another example for both Grand Theft Auto and Grand Theft Auto 2. I've spent 25 minutes with GTA and 44 minutes on GTA2. I feel like I could do an article for the first game, again because I feel like I know what is expected of me in the game. I won't presume to know everything about the game or to be an expert, but I have the general gist of what's going on. In GTA2, I feel like I need to put in at least another hour or two before I could write something that sounds like I know what I'm talking about, which I will probably do in the next few weeks, but don't hold me to it as I recently put Mario Kart 7 (I know, I'm late with the games again) in my 3DS and Steam always seems to have 50-75% off sales on games that look like they're too much fun to pass up.
I feel it all comes down a fraction of the amount of time it would take in order to complete that game. What is that multiplier? I don't know and I don't think that it could be whittled down to a simple percentage. So I just invalidated the first sentence of this paragraph, which any high school English teacher is a bad way to write a paper. Thank goodness my sister, who is a high school English teacher (and our mother who was an English major in college) aren't frequent readers or I'd frequently be hearing from them about improper syntax or using multiple tenses of verbs in the same sentence. But this is all beside the point.
Now, I can't speak for the Good Doctor (English major) on how he determines when is a good time to write up a review or an impression, but I feel that we're at least reading the same book, if not the same chapter.
Stage 1 of the Process