I’m going to state what might be the obvious here. The more you practice, the better you get. But it’s a bit more complicated than that. Like with any endeavour – whether you want to become a pro bodybuilder, or a math champion, or anything for that matter — it’s simply not enough to practice once a week or so.
Ultimately you must think of building your tennis game as if you were building a skyscraper. How high you build yourself depends on how quickly and routinely you go. If you only place one brick in the ground every week, by the end of the first year, you’ll still be working on the foundation and it’ll be likely falling apart all over the place.
I made a little chart above to demonstrate the relative payback from practicing 1x per week vs. 3x and 6x(green line). The Y-axis represents skill-units, and the X-axis represents each week over the course of a year. If you practice only once a week, you will constantly be struggling to build the foundations of your game, and just when you have learned one thing down pat, you will forget another, so you will plateau very quickly having never truly learned the basic foundations of the game.
If you play 3x per week, you will improve much more rapidly as you will be able to build upon the foundations you learned from the last practice, but you will still frequently forget certain skills and the progress will be much less parabolic than were you to practice 6x per week. The key is that by increasing the frequency of your practice, you do not forget what you learned before, and waste time learning and re-learning the foundations.
If you can’t find the time to play that frequently, then what you should do is get a small notebook and write down everything you learned during a practice. Periodically review your notes so it stays at least somewhat fresh in your mind. This in no way replaces practice since you aren’t learning anything new. It just helps prevent you from forgetting too much.