I started playing in 2014, got bored and left, then got curious again when I heard of Plains of Eidolon and Fortuna. Resumed playing in late 2018, and haven't missed a day since. Still have a lot to learn about late game, but I think I'm fairly knowledgeable of the basics by now.
Edit November 2019: It's been about a year since I came back and I'm already into late-game. I'm at that point where most of what I have to do is grind out MR fodder and fine-tune my FashionFrame prowess. Still not an expert, but I think I've got a good handle all-round.
Edit February 2021: I've reached the point where I have rivens on a lot of my most-used weapons. Putting forma on things happens more often than ranking up new things. I've found my playstyle and have gotten pretty good at applying it to most frames. I'm not an expert by far, but I'm extremely well versed in how I play the game now. Next hurdle is overcoming complacency and finding my ambition.