Fitbit expires_in oauth2 token is always 30 days

Question

I'm using Implicit Grant Flow to authenticate over oauth2 and obtaining token from fitbit platform. Here is the url I'm requesting:

https://www.fitbit.com/oauth2/authorize?response_type=token&client_id=CLIENT_ID&expires_in=31536000&prompt=none&redirect_uri=REDIRECT_URL&scope=activity%20heartrate%20sleep;

Since I'm specifying expires_in=31536000 I'm looking for a 1 year validity token, but I'm receiving a response like this:

fitbit://mypersonalcallback#access_token=ey...VE&user_id=blablabla&scope=sleep+activity+heartrate&token_type=Bearer&expires_in=2567006

So my token lasts 30 days. How can I obtain a 1 year token?

Thanks!