Is there an estimated timeframe on approximately when this will happen?

Isn’t it already happening? Is that what is creating the huge methane explosions and craters in Siberia?

Yes. Check the Yamal Peninsula in Google Maps and you realize this kind of cratering is a process that has been going on for thousands of years (since the ice age really!).

The scary part is that it's accelerating in on a timescale that's relevant to our needs as a species.

I think the largest deposits would take thousands of years to be affected, but there are shallow deposits that could be released, especially those near glaciers, where the glacier melting allows the nearby seabed to rise.

https://en.m.wikipedia.org/wiki/Clathrate_gun_hypothesis

What kind of quantity of newly released methane are we talking about, compared to the methane produced by trees and other plants?[1]

1. https://www.npr.org/templates/story/story.php?storyId=515109...

1% of the carbon stored in the permafrost is roughly as much a all of humanity is currently emitting per year. It's a lot. Permafrost thawing is an important tipping point in the climate system.

Yeah, it's a lot. https://en.wikipedia.org/wiki/Arctic_methane_emissions

See also

https://en.m.wikipedia.org/wiki/Clathrate_gun_hypothesis

Could we just replace the ice with concrete (before it thaws)?

And that, schoolchildren, is why Siberia is the world’s largest roller rink.