Checks every 10 seconds
The fastest interval is now 10 seconds, from 15. It is available on hosts under a verified domain, on accounts that have bought credits at least once. At that interval a monitor costs 259,200 credits a month. Across all accounts a single host is still checked at most 12 times a minute, so the fastest intervals can be unavailable for a host that several accounts monitor. The home page calculator and Intervals and credits have the numbers.