It depends on network but approx 20 secs for ETH transactions and roughly 10 mins for BTC.
Miners are paid for their work validating transactions and adding blocks to the Ethereum blockchain in fractions of ether (ETH). These fractional units are called gwei, and comprise the gas price for the transaction.
If a transaction needs to be confirmed urgently or as soon as possible, a higher gas price should be included with the transaction.
It is important to communicate to the miners how much work needs to be done in order to process a transaction. This is done with the gas limit, which the Blockchain Wallet calculates automatically to ensure that transactions go through successfully. The limit also prevents overspending on mining fees.
If the gas limit is set too low, a transaction can fail, or get rejected, which would result in losing the gas paid for the transaction. If a transaction is processed before the limit is reached, the rest of the gas will be returned to the sending wallet.