Since 14:05 UTC, Check every on an Enhance, cPanel/WHM or Plesk connection offers every interval a monitor can have: 1 hour, 30 minutes, 15 minutes, 5 minutes (the default), 1 minute, 30 seconds, 15 seconds and 10 seconds. Before, it stopped at 5 minutes.
A slower interval suits a server full of small sites that only need a daily sense of «still up», and keeps more of them inside the free credits: at 15 minutes, 225 sites fit in the 650,000 a month; at 5 minutes, 75. The page shows the monthly cost of each choice for your number of sites.
The page also says what the setting is: the real interval of every site of the connection, existing and new, not an estimate. A single monitor can still be set on its own.
Thanks to Press Wizards for asking.