I think D P Davies in his classic book "Handling the Big Jets" (written in the 1960s but still has some great guidance) states that the probability of an actual stall in commercial operations was one in ten million (or was it one million - would have to check?) but the probability of a stall warning was much higher at one in 100,000.
Obviously it is important that pilots know how to recover from a stall but far better to prevent the aircraft from stalling in the first place. Notwithstanding stall warning systems this requires a knowledge of the a/c performance and also correct monitoring of vital parameters such as pitch attitude, airspeed, power etc.
If you see ten degrees or more pitch up at cruising altitudes in a swept wing jet transport you better start making a correction before you get a stall warning.
Even before an a/c stalls and/or the stall warner operates (assuming from level flight or thereabouts) the first warning signs are low and reducing airspeed, high nose attitude and reduced control effectiveness.