Switch off (curtailment) is almost instantaneous for wind and PV.
And otherwise, batteries, hydropower and gas turbines are dispatchable on the second/minute scale.
How dispatchable nuclear is depends on the specific design, which is important given our context of small reactors with new designs.
According to the DOE, "Certain designs, like DOE-supported NuScale Power, LLC, can vary their energy output over days, hours and even minutes."[1]
According to NuScale, "The NuScale Power Module is capable of a ramp rate of 40% per hour in reactor power change, which aligns with specifications set by the Electric Power Research Institute (EPRI). For even quicker responses to electricity demand, the NuScale SMR can rapidly lower its electric power output up to 10% per minute and return to full output at the same rate utilizing turbine bypass. This is significantly faster than conventional nuclear power."[2]
More advanced designs may be even better.
[1] https://www.energy.gov/ne/articles/department-energy-report-...
[2] https://www.nuscalepower.com/-/media/nuscale/pdf/publication...