You get to decide based on how much transaction fee you're willing to pay to the miners that confirm your transaction. The simplistic explanation is that the B

You get to decide based on how much transaction fee you're willing to pay to the miners that confirm your transaction. The simplistic explanation is that the Bitcoin network effectively incorporates a hard upper limit on the number of transactions per unit time. (Every new transaction must be recorded in the blockchain, and the blockchain only grows so fast.) So as the currency becomes more popular and demand for transactions increases, either your waiting time to get the transaction confirmed on the blockchain increases, or your transaction fee has to increase.