I see that you wrote this post 4 months ago but I'm going to reply anyway! :)
I too have been in Italy for 4 years (I'm spotting a theme here ha ha!) and have learned Italian purely by listening and talking. In fairness It's got me a fairly long way and like you, I'm able to make myself understood and I understand pretty much everything that is said to me. That said, I have had no formal training and therefore my confidence in what i'm saying is pretty low. I'm terrified of arriving at the 5 year mark and still not being able to communicate fluently so here I am!!
I see nobody has responded and i suspect that is because it's an impossible question to answer. It very much depends how much Italian you already know and how much you study every day. I've decided to start at the very beginning in case i've picked up any bad habits (I'm not sure if you've noticed but Italians are the first to admit that they don't always speak Italian very well!!) and have reached the middle of module 2 in only 2 days without dedicating much time which is probably much faster than someone who has never spoken or listened to Italian before in their lives! Our situation is probably very different to most other people here in that we're surrounded by the language and have a million opportunities to practice every day.
I'm curious how far you've got since you posted and have you are finding it?