Depends on where you are, what infrastructure is in place, level of automation and human reliance e.t.c. there was an old show that had some info on this, called something like "days after we're gone" or something. They brought up that certain levels of the grid for certain nuclear reactors could potentially function for years, even decades. While most coal power plants and battery/capacitor charges would be gone after 2 or 3 days. Renewables could last years if solar, as long as someone cleans off the panels.

All that isn't even mentioning substations, if those go down or flip a breaker (which is likely to happen within a week or month) that area of the grid is down and it can make it harder on the rest of the grid when it does, unless the grid is working in a power deficiency.

Point is it's fuckin complicated lol. Get a generator, or if your rich some batteries and solar panels.

Based on my relatively surface level understanding of electrical grids, a couple milliseconds maybe. Depends how automated supply and demand are balanced.

The issue isn't necessarily the physical infrastructure, it's that electricity generated now must be consumed now. Without people to help manage this very delicate balance the system doesn't work.