Is 5,000 USD enough for 3 months?

So I will go into as much detail as possible. I am planning to go to Japan when Available to visit my Boyfriend for 3 months. I have about $5,000 USD in my saving for the trip. I will be staying with him and wont have to pay for hotel or anything like that. Also I already have an airplane ticket purchased so any of my savings wont go to that. He loves to cook and of course Ill help pay for food but hes already got at least dinners covered. My concern is, will immigration let me into the country? I've heard stories of people not being let in due to money funds. Any Knowledge on this would help alot. Thank you.

$5,000 is more than enough. Immigration will not stop you. They rarely ask, especially if you are staying with your boyfriend.

I have been let in with A LOT less. Also immigration have never actually checked how much money I have on me.

Never been asked about money. And who carries cash nowdays? I always enter Japan with 0 Yen but 2-3 credit cards.

I would be more concerned about the airplane ticket you have already bought. Are you sure you're able to get on the airplane on the day of your flight? Where are you flying from?

The flight was canceled but I got an e-credit for the next time I fly. Its good until 2022. Thank you everyone for the helpful answers.

I've heard stories of people not being let in due to money funds.
Stories - usually there is more to it than just that if there are actual cases and not just some rumour on the internet. When I was travelling/commuting to Japan I usually had zero cash. I've never been asked about money in decades of regular travelling between various countries.
As for how much you need, if you get through half of your savings you're possibly spending too much, but if you like going shopping and do short holidays out of town you can get through a bit more.