Hey Shanz!
I think that it depends on how you're doing it, and if you're prepared in the ways of strength and technique! It took me about four years to invert, but I was doing a lot of "off time" from pole through those years- it took me a long time to build up the strength.
You can always check out Veena's lessons, she has a really good Strength routine that will help with inverts, as well as Dirdy Birdy's lessons on Youtube, where she also gives some very similar tips on preparing for the Basic Invert.
You are right, one should NEVER jump into an invert! It can really damage the muscles in your arms, shoulders and back. It's good that you're watching for that 🙂 Otherwise, just try to watch your technique, and ask a lot of questions! Your instructor is there to teach you, and should (hopefully!) be well versed in the ways of muscles, of pole dancing, and the risks therein.
I've heard of people getting into inverts about 3 months in, 6 months in, 1 year in… and occasionally, 1 month in! But that last is a little scary, if they're new to the journey of strength and fitness.
It's all about you, and how your body reacts! If you feel strange or uncomfortable doing it, I would say not to do it. You can always try the "second half" of the invert by going at it from the floor, and eventually working your way up. There ARE other ways to get upside down (my current preference is the Cross Knee release) but they will also get you into different possibilities, and block others off. It's good to have a full repertioir. If you're afraid of being upside down in general, you can also try those moves for somewhat more stability (depending on which you prefer, of course!)