If your transaction size is smaller than 1,000 bytes, all outputs are 0.01 BTC or larger, the transaction is of high-priority (https://en.bitcoin.it/wiki/Transaction_fees#Technical_info) and all your inputs are confirmed, the transaction should go through pretty soon even without a fee.

From my own personal experience, slightly more than half of them get included within the next 3 blocks and almost all of them get included within the next 6 blocks.