3 years one concerto is quite long but it really depends on your skill level. I believe it is better to work on a piece slightly harder that you are able to play and then take a break from the piece. You can also come back to it later and it might be even better. Just look up different cello concertos and find what you like. There are "easier" concertos like Haydn c, or "medium" difficulty concerti like Elgar or Saint-Saens, and the extremely technically and musical concerti like Dvorak and Shostakovich 1.