★http://support.f5.com/kb/en-us/solutions/public/10000/600/sol10640.html
Note: F5 recommends setting the timeout of custom monitors to three times the chosen interval time, plus one. This particular ratio ensures that more than one health check must fail in order for a pool member to be marked down. For example, with an interval of 5 seconds and a timeout of 16 seconds, the BIG-IP system must not receive any valid response from a pool member for 16 seconds before it can mark the pool member down. Because in 16 seconds the BIG-IP system will run four health checks against the pool member, all four health checks must fail for the pool member to be considered down. Ultimately, this configuration prevents the status of a pool member from flapping unnecessarily.