That is a good question, and the answer is not simple. A necessary conditions for water vapour in a given air-parcel to condense out is that relative humidity s

That is a good question, and the answer is not simple. A necessary conditions for water vapour in a given air-parcel to condense out is that relative humidity should be 100%, or the less known concept of a dew-point temperature is the same as ambient stratospheric temperature. In other words, it is possible that H2O vapour concentration did not reach saturation levels at the given air temperatures (stratosphere). However, due to passage of waves through that region it could, locally, trigger condensation. However, the actual micro-physics of having enough condensation nuclei, the physics of growth and falling of micro-dropplets, and large scale circulation and winds complicates to estimate the actual time scale of residence against condensation. Even the original article does not try to touch any of these aspects, rather create a 'narrative' without any quantitative (even back of envelope) estimates. But one has to appreciate that 145 Tg of water injected into the stratosphere in a single episodic release is mind-blowing number.