Per the description:
Wards an ally with damage reduction 10/adamantine. Once the spell has prevented a total of 10 points per caster level (maximum 150 points) it is discharged. Otherwise it lasts 1 minute/level.

A level 16 caster casting stoneskin lasts 15 minutes. An extended stoneskin from the same caster lasts 30 minutes. Both only prevent 150 damage.

Perhaps the wording should reflect:
Wards an ally with damage reduction 10/adamantine per caster level (max 150) for 1 minute per caster level (max 15.)