I am going to go against the grain here and say that there really isn't a number that you should be aiming for. You need to always be improving retention regardless of what number you are at.
Also worth noting is that performance due to retention varies wildly from video to video, niche to niche, audience to audience and even by total video length. For example, if you have a twenty minute video and you are getting 40% retention that video is collecting more watch time than a 10 minute video with 70% retention.
Endscreens are also an important element here. It's not just about retention, but how many people get to the endscreens and suggested videos at the end of yours. The more clicks you get on suggested videos and endscreens the better that video is going to perform. That is why it is a good practice to allude to something in your video that causes the viewer to stick around for the whole thing.
For example if you are talking about coins, you could mention one of those coins and say something like; 'now this coin... this coin is something different entirely. But we'll get to that in a minute.' Continue talking about the other coins, and then close the loop by going back to this mysterious coin and why it is different in the end of the video.