it is changed in compile time

we can't change module attributes and endpoint settings in runtime

lib/pleroma/web/oauth/token.ex

@@ -14,7 +14,6 @@ defmodule Pleroma.Web.OAuth.Token do
   alias Pleroma.Web.OAuth.Token
   alias Pleroma.Web.OAuth.Token.Query
 
-  @expires_in Pleroma.Config.get([:oauth2, :token_expires_in], 600)
   @type t :: %__MODULE__{}
 
   schema "oauth_tokens" do
@@ -78,7 +77,7 @@ defmodule Pleroma.Web.OAuth.Token do
 
   defp put_valid_until(changeset, attrs) do
     expires_in =
-      Map.get(attrs, :valid_until, NaiveDateTime.add(NaiveDateTime.utc_now(), @expires_in))
+      Map.get(attrs, :valid_until, NaiveDateTime.add(NaiveDateTime.utc_now(), expires_in()))
 
     changeset
     |> change(%{valid_until: expires_in})
@@ -123,4 +122,6 @@ defmodule Pleroma.Web.OAuth.Token do
   end
 
   def is_expired?(_), do: false
+
+  defp expires_in, do: Pleroma.Config.get([:oauth2, :token_expires_in], 600)
 end

lib/pleroma/web/oauth/token/response.ex

@@ -4,15 +4,13 @@ defmodule Pleroma.Web.OAuth.Token.Response do
   alias Pleroma.User
   alias Pleroma.Web.OAuth.Token.Utils
 
-  @expires_in Pleroma.Config.get([:oauth2, :token_expires_in], 600)
-
   @doc false
   def build(%User{} = user, token, opts \\ %{}) do
     %{
       token_type: "Bearer",
       access_token: token.token,
       refresh_token: token.refresh_token,
-      expires_in: @expires_in,
+      expires_in: expires_in(),
       scope: Enum.join(token.scopes, " "),
       me: user.ap_id
     }
@@ -25,8 +23,10 @@ defmodule Pleroma.Web.OAuth.Token.Response do
       access_token: token.token,
       refresh_token: token.refresh_token,
       created_at: Utils.format_created_at(token),
-      expires_in: @expires_in,
+      expires_in: expires_in(),
       scope: Enum.join(token.scopes, " ")
     }
   end
+
+  defp expires_in, do: Pleroma.Config.get([:oauth2, :token_expires_in], 600)
 end