I suspect hours or minutes.
Seismic stations should have been able to read the disturbance and armed with that information folks should have been able to make some pretty accurate determinations.
My thought is, why is this being lofted now? What goal does that serve?
We well knew the seismic disturbance after a couple of weeks.
Determining the blast yield on an underground test is tricky.
There were 3 parts to the blast, conventional detonator explosion, followed by a supposed fission explosion and a third disputed explosion which may have been an attempted fusion explosion.
The timing closeness had to be separated to look at the individual blast yields.
Good study on blast yields here:
https://apps.dtic.mil/dtic/tr/fulltext/u2/a223490.pdf
Lastly and most importantly, we have little data on the rock and soil density of the blast chamber, just some very old mining surveys. That greatly affects the seismic measurements.
There was very little venting to sniff for byproducts of fission and possibly fusion.