A health/damage mining system would be much more flexible and should be easier to implement than a fixed, time-based system. Basically, assign each block a health value, and your pickaxe a "mining damage" value, and whenever the pick strikes the block, you subtract its damage from the health of the block. When the block's health goes to zero or below, the block is mined. In order to achieve a certain time span until a certain block of ore is mined, then set the block's health equal to damage per pick strike * pick strikes per second
Advantage: You can re-use the existing health system for all your other entities, if you have one, and you can much more easily introduce and balance kinds of ores that are easy/hard to mine and pickaxes of different strengths. You can also give your players more flexibility in what tools they use for different tasks. For example, why not allow your players to mine ores with dynamite, i.e. explosive damage, instead of only pickaxes? A health/damage system allows you to do that quite easily.