I just finished the game, I'm a bit of loss for words right now. This game really does deserve to be looked at as one of the best games on this site.
For me there are 3 things which I look for most in VN games, the visuals, the story and the characters. While I'm not a massive fan of the honey select style in game, all the characters look great, the animations are well done, and subtle things like raised eyebrows or eye movements are done really well and bring life to the scenes.
The characters are all unique and likeable. The relationships grow over time and feel for the most part natural. Its really hard not to like any of the main characters in the game. The MC is also likeable, which is very important as you're playing as him!
The story just blew me away. It was interesting until the end, but it also made you really care about the characters in the end. No spoilers here, but there were a few times I'm not ashamed to admit I teared up a little at times during the last few scenes and the ending. The ending felt natural, not rushed, well paced, which I find is a big problem with a lot of the games on here, they're not sure how to end. This game's story flowed perfectly in that regard.
The writing was very good, and quite funny, with lots of humorous lines and renders. The overall presentation was great. The music and sound effects definitely added a lot to the overall experience and are very well done.
A great story is something that can invoke an emotional response in the viewer or reader, making a connection with the characters and their journey. Once in a Lifetime does that beautifully. I can't really say any more. One of my top rated games for sure. If you haven't played it, please do!