A thirty-year architectural shingle is rated in a laboratory. On a south-facing mountain slope with a hundred freeze-thaw cycles a winter, the real number is lower.
Ventilation sets the ceiling
A properly vented roof gets close to its rating. A badly vented one loses a third of its life, reliably.
Pitch and aspect matter
Steep north-facing slopes outlast shallow south-facing ones every time, on the same house with the same shingle.