You need 100 confirmations before you can spend coinbase coins. Today most blocks are full, and have between 1000 and 3000 transactions in each block. If we take an average of 1500 transactions in each full block, then spending a coinbase coin as fast as possible would require waiting for 100 blocks with an average of 1500 transactions in each block. That makes 100 x 1500 = 150000 transactions in the chain going back to the coinbase of freshly mined coins spent as soon as they possibly can be.