As knightdk mentioned, the Bitcoin Core client tells you that it may take up to 25 blocks to confirm if you leave the default setting.
You might get lucky and have it confirmed in the next block, but if it ends up taking the entire 25 blocks at an average of 10 minutes per block you should expect it to take up to 4.5 hours to confirm.
If you just started up your client and it hasn't had time to get a good sense of current confirmation times before you send your transaction, then it could estimate incorrectly and take even longer than that.