The 10:20 system is an effective technique used by lifeguards all over the world. It requires scanning from one side of the pool to the other, or the designated area, in 10 seconds. The second requirement is that the lifeguard be no further than 20 seconds away from any swimmer who may get into difficulty in the lifeguard's area.

The 10:20 system is taught to lifeguards on many courses including the UK National Pool Lifeguard Qualification

Citation: BSA Lifeguard Manual