There is two types of testing. You did not specify which you talk about.
Testing for bugs. Traditionally, you do tests yourself till you are satisfied and move from alpha test to beta test and if you are confident enough, from beta to release. But this is not a value, like let's search x hours for bugs. You just test the game in hopefully all scenarios and eliminate all the bugs you can find, before any scheduled release date. For bigger titles, that got sloppy over the years.
Testing for balance and how the game is liked by the target audience. If you have no balancing, the answer is obvious. But you should still collect feedback by honest testers. Your proverbial mom probably is not unbiased. And while friends might be honest with you, they might not be the target audience, or might not be able to pinpoint what is missing or too much. If you can release a demo and have an audience, that is an ongoing process, and the art is to hear the valid critique, even if there are vocal minority opinions.
Trivially, the only person needed is you, and the time needed for testing is 0, because it happened while the development was happening. The other end is what you said, about never enough. To draw a line, you would need to have some type of constraint. Like a release date. Since we are on Itch, that's not an issue. If it was about bugs, just have a few playthoughs been done, and if there are no game breaking bugs, it should be good to go and not be ashamed to have status "released" instead of "in development".