From the day I get my starter till the day I finish the league, I would always have my starter in my team. Always. After that it would have to depend what I want to do with the rest of the game, GENERALLY, my starter will still stay with me. But sometimes I either trade him/her to a friend so they can make the most of that Pokemon for their team. Or I would put my pokemon in daycare and catch a new Pokemon for my team and start a little adventure to level it up.