When do headgaskets blow or engine block crack at what mileage is normal?

372,834 miles on my '95 Camry , NO smoke , no blown/cracked head yet!! Unless there is a DEFECT , or other known fault to CAUSE it , I don't consider any mileage 'normal' for headgasket or block issues.

103,300 corrion on block. Pea size cavity near through bolt, only visable once starter was removed. I repaired it myself for $59.