It sort of depends on the definition of "abandoned their post". If they shut it down on the way out, about 15 seconds if coal is involved.

If they don't shut it down on the way out, you'll run in to issues as soon as the load no longer roughly matches what is generated, either way. This is because the entire system is designed under the assumption that these will roughly match. If they don't, generators and other equipment can end up destroying themselves. This imbalance will almost certainly happen within about 12 hours. Everything would start catastrophically failing sort of like as happened with the Texas grid a year or so ago (though they got that under control before it was an enormous issue)