The reward is always halved every once in a while so theoretically it would take forever to mine every single coin in existence but because of technical limitations, there might be a limit to how many digits the lowest denomination can have. In that case, the last coin will be mined and the full supply of coins will be in circulation. However, that's not the only way miners make money. They also get miner fees, and as the reward goes down, they go up. So nothing will change much in the end, maybe the coin prices will go up, just like how it usually is with every halving.