How long did it take you to get to 1K, 5K, 10K, 20K+ Followers?

in the begiing you gain between 2-4k per month, the first month might be a more around 2k if you start to increase your activity slowly, i.e. warming up your account.

the more followers you have, the higher your monthly gain tends to be.

Since this depends on the niche, you can't really say how long it takes. some accounts grow faster than others.