Generally pretty often, frequently the hit from a NLAW or Javelin from above triggers the ammo, which then blows the top off, and kills everyone inside. The ammo forms a ring around the turret, and once the ammo goes off it often flings the turret off the tank.

The Russian tanks are smaller and use 3 people, and an autoloader. But the ammo is stored in the crew compartment. The American M1 abrams is larger, has a human as a loader, and has a separate compartment with blow out panels so the ammo can cook off and the crew could survive. I've no data on that survival rate, but presumably it's much higher than the Russian tanks.

Semi-related but it's interesting to read about the Merkava and its design philosophy, because crew preservation is the number one priority.