My heuristic for improvement is: six weeks, six months.
Need to get in shape? (Whatever "in shape" might mean, but that's another topic.) It's going to take six weeks before you notice any changes, before any benefits are apparent. It's going to take six months before you're really building strength or endurance.
Six months or six weeks aren't real, scientific numbers—they're just heuristics. When you start some habit, it takes a solid starting chunk of time to lay the groundwork for improvement. When the habit is solid, it takes another, larger chunk of time to get the results of the habit to some satisfyingly high level. Six weeks and six months are close enough to capture that.
I am relatively out of running shape now (at least compared to past exploits). I was building it up this year, but the last few weeks didn't leave much time to maintain that. OK. Let's pretend it's a start from zero. How long is it going to take to solidify the habit? Six weeks. How long is it going to take to get in competition shape? Six months.
They're both daunting numbers. There aren't any shortcuts. You just have to keep doing it until you're better at doing it. There are matters of technique, coaching, learning, each of which have their place in improvement. But the real magic is buying into maintenance —relentlessly keeping at it until you get to the threshold where you can take on something more and learn to do it better and better.