As far as we can tell from the geologic record, they can collapse quickly. How fast? Don't have good data on that.

But some of the floods from the Laurentian Ice sheet, itself kilometers thick, were pretty huge. Some studies of the flooding of Doggerland (https://en.wikipedia.org/wiki/Doggerland) was of the sea rising very fast, enough to noticed in months.

Glaciers don't just melt from above, they melt from below, as surface melt sinks to the bedrock. There, it doesn't just melt the ice above, it lifts it. Hydrostatic lift. Not lubricate the bedrock ice interface, lifts the glacier up. And there are huge pockets of water under that ice right now.

The old Mile High Stadium in Denver could convert from football to baseball by moving a huge structure on a thin film of pressurized water. (https://www.thedenverchannel.com/sports/broncos/when-mile-high-stadium-floated-on-water-the-broncos-old-home-transformed-in-hours).

But Greenland will collapse first.