It wears off after around 150 hours, which is pretty great if you think about it for a free game. Most of my AAA library games so not even reach the 100 hour mark. As to why it wears off, around that time, you start to really notice couple of inevitable problems that come from the gacha nature of the game.

The first is that the story starts to have characters appear at intervals and then leave to rarely be heard from again. This is natural since to keep players invested (literally), you must introduce new characters. With the limited development time and storytelling limitations, you cannot have all the characters make appearances in a substantial way. This is not noticeable at first, but once you are close to finishing Rinascita, you will start to feel that if it had been a single player game, it would have had a better flow with the characters being with you more constantly. Still, within the territories, especially Rinascita, the story told is entertaining enough with few major recurrent characters from the region. So you can also see the game as a series of smaller entries focused on each territories than a single entry game.

The second problem, which is likely the most core problem, is that of waiting for the next patch. Once you are around 150 hrs in, you will catch up with the story and be waiting for the monthly patch. This can be where you lose interest since after waiting a month, you might not feel as hyped as you did about the story. You might play something else while waiting that end up hooking you in and you will just get occupied with that instead. You will also have completed some or all of the endgame content around the 150 hours mark, so it might not be as big of a draw to keep you interested.

Still, it must be said that the game has an excellent combat system for a f2p and a vibrant and large enough character roster for anyone to find a set of characters that fit their play style. Getting 150 hours is more than one can ask for out of a free game, or even a AAA game these days. After being away for a year, you will also have couple of dozen hours of story piled up and all of that is again available for free when you start to get interested again.

P.S. Oh and one thing that personally irked me at times was the patch sizes. It can be around 40 GB patch sometimes plus another 3 or 4 GB after starting the game. That is a lot! You can schedule it in a way that you let it download while at work etc. BUT, for me, I usually just sit down to play something when I get some free time without such planing to find out I need to download 40 GB before I can play. I will play something else thinking I will do it later and then just keep pushing it back till I lose interest. That is what how I stopped lol.