Confirmation is something that essentially provides proof that something is indeed true. Think of this as when you receive confirmation that your Uber ride will arrive on time, or that you find out that something you thought might have happened, has actually happened.
Confirmation is essentially a verification or final proof of something.
In the world of cryptocurrencies, confirmation is a measure of how many blocks have actually passed since a transaction was added to a coin's blockchain. The more confirmations that have occurred, the more secure the transaction actually is.
When it comes to smaller transactions, buyers and sellers can agree to wait for zero transactions, while for larger transactions parties could potentially want to wait for more blocks.
When we discuss Bitcoin specifically, three blocks are a good safe middle ground for confirmations, while six confirmations are advised when it comes to larger transactions, and for every large transaction, you might wait for 60 confirmations.
Each merchant, as well as the exchange, has to decide how many confirmations they require for each coin. There is no specified number when it comes to this, and the number of confirmations that have to be 100% safe increases over time.
When it comes to the length, it depends on the coin itself, as each coin has a different time frame for how quickly the blocks can be mined.
With Bitcoin, a block is added about every 10 minutes or so. This means that there will be one confirmation every 10 minutes, which starts once an unconfirmed transaction is added to the blockchain itself.
Keep in mind that an unconfirmed transaction is one that is waiting to be added to the blockchain by miners. Each block is then added after that initial block and results in one confirmation. This means that if one confirmation takes 10 minutes, six could take an hour, and so on.