I've had a striker I bought in L2 that made it through to the end of L12 with the help of Personal Trainer so spent 11 seasons at my club before being sold. Same went for a MC that spent 8 seasons at my club. (Yes back then I didn't know the effectiveness of power training until L13, but these were all farmed tokens I saved up that I used.)

Currently I have 5 players serving in their 6th season at the club: Papaleo, L.McNamara, Mensah, Alex Zidane and Novotny and those all are joint-holders of my longest serving player at the club. Novotny and former 9* player Alex Zidane will be sold at the season end this season or the beginning of next season with both L.McNamara and Mensah both will most certainly leave the season after as these players are declining, Papaleo will probably be gone the season after that.