It depends. Some videos have a short life and some videos are evergreen. A video about a news topic, for example, would be a video with a short life. A video about a 4th of July fireworks display would also have a short life. A DIY video about a common problem people face would have a very long life. If you have evergreen content that at one time was getting a lot of views and now is dropping, the first thing to do is to try improving the SEO of the video. Changing the title, thumbnail, tags, description, is a way to maybe generate some new interest. This takes time as results may take months to materialize. You just keep tweaking and waiting.
Three factors affect views.
1. Search Result: The higher your video ranks in the search results, the more often people will see your video, which increases the chances of them choosing to watch your video. The SEO of your video directly impacts where your video shows up in the search results.
2. YouTube Suggestions: There is almost nothing you can do on your end to affect when and where YouTube Suggests your videos. YouTube uses things beyond your control to determine this, and many times YT just does what they want regardless of anything you do. Many here will disagree with me on this, but all statements will just be unfounded opinions as no one has any real tangible solid proof to present to support their opinions on how and why YT suggests things. It is all subjective conjecture. To be clear, my opinion on this is subjective conjecture as well, so take it for what it is worth.
3. Thumbnails: Getting your videos seen in search and suggestions is only the first step. The viewer still has to click on them. Nothing has a more direct impact on whether people click on your content than the thumbnail. As the old saying goes, "First impressions are everything". Your thumbnail is going to be the first thing a viewer will see about your video and in 99% of the cases, it will be the determining factor as to whether or not they click on your video. Opinions on what is a good thumbnail are as vast as the universe. The only thing you can do is test out different thumbnails. If you want to pay the $49 a month for TubeBuddy Legend, TubeBuddy can do A/B testing on your thumbnails to determine which is the best, but you can do this yourself with a little effort. Personally, I think Legend is way overpriced for what it offers when you consider that you can get Abobe Creative Cloud for only $53 a month and get unlimited access to every top-level professional program Adobe offers. But if you have money to burn, it is there if you want it.