Block Time

Bitcoin's target block time is 10 minutes. That's the average interval difficulty tries to maintain by adjusting how hard the mining puzzle is. It's not the time you should expect to wait for any individual block.

Mining is a Poisson process. Each second, every miner in the world is independently trying random nonces. There's no "due" block. The actual interval between blocks is exponentially distributed: many blocks land in 1-5 minutes, some take 20-40 minutes, and a few times a year a block takes over an hour. The 10-minute number is just the mean over a long window.

The choice of 10 minutes (rather than, say, 1 minute like Litecoin or 15 seconds like Ethereum) was a deliberate tradeoff:

• Long enough to let a new block propagate to virtually every node on Earth before the next one is found. This minimizes orphan blocks and chain forks.
• Long enough that the proof-of-work per block is meaningful security.
• Short enough that confirmations accumulate at a useful rate. Six confirmations (the conventional "settled" threshold for large amounts) is about an hour.

The difficulty retarget every 2,016 blocks pulls the average back toward 10 minutes when global hash rate has grown (or shrunk). See the Mining rabbit hole §4 for the long version.