You need to be ranked top seven in your league.

You can only level up once per season, which takes 28 days.
As such even if you level up each season it will take two years to reach level 25.

To experienced players, leveling up is actually undesirable because the competition became harder, but its inevitable if you want to win a league or play in champion league. There are players that completely abandon league and CL just to became competitive in cup.