Critical mass in this sense means what it will take for bitcoin to sustain and grow to become the most used currency.
Is the critical mass 1% of all transactions? 0.1%? 0.01%?
Most used currency where? The US? The First World? The whole World?
It is pretty strong now, 4 years without being hacked, market cap having crossed a billion. No question it's growing.
We have to spend much more time formulating the question before we can begin to answer it.