The term block time relates to an average time. It is a measure of for how long it takes the hashing power of the network to search a solution for block hash. The difficulty is calculated as proportional to the hashing power accordingly, on an average, the block time is 600 seconds for Bitcoin and 20 seconds for Ethereum. In reality, block time can vary from one to another. In layman terms, Block time is the duration which the networks take to generate one extra bloc. For every cryptocurrency, block time varies.