John Fisher bought the A's in 2005 and ran them into the ground so he could move them in 2025. That was 15 years. Also, come 2040, Target Field will be as old as the Metrodome was when the Twins left it.
However, I see very little chance the Twins are moving in 2040 as relocation is MLB's biggest threat to get new stadiums. The other owners aren't going to let the new guys in the club jump in line and steal their best leverage to improve the stadiums that actually are outdated. The Coliseum in Oakland was already a terrible stadium in 2005, and like the Coliseum, the Metrodome was ALWAYS a cut-rate stadium that didn't match up to it's peer venues. Target Field still ranks as one of the best in the league.