There is no sane condition in Redis that will make it not replying at all AFAIK, even if you set maxclients to 1 the next clients will have an error returned (and the connection closed ASAP). But yes, it is important to understand what down means. I think one of the safest things to do is "down == unreachable". So if you don't get any reply at all, for the configured amount of time consecutively, the server is down. And of course the other redis-sentinels have to agree for the fail over to start.
The gem has a configurable --max-failures option that can be passed to failover daemon. The daemon will only mark a node as being unreachable if it fails to ping that amount of times (default 3). This might be something that can be improved too, but it was meant to avoid false positives.
