Good question~
I've been doodling for about 12 years, properly drawing for 2 years (digital media) and for the last 3-4 weeks I've been trying to do more than one complete drawing a week and build a career, and I think I'm still defining my style lol. Every time you think you've mastered something a completely new thing to master comes up. In the beginning you can pick a lot up quite fast, learning good hand control, autonomy, basic shading all at the same time.
For me I then started looking at shading in more detail and I'd sort of got my head around drawing in 3 dimensions (recently) at the same time. Both of these I think have totally evolved my style again. It seems never ending lol.
Here's my first ever drawing of my fursona in 2013 compared my most recent one. You can sort of see how my style developed. I was worried for the first few years I didn't have a style, but I think everyone does and shouldn't worry about it. If anything, trying to force yourself to make a style I think can actually stop you developing a style that fully reflects you. But this' just what I think, not to say it's true.