Well.. it's certainly been a long time since it ruptured big..

Southrn Calif Earthquake Data Center says:
LAST MAJOR RUPTURE: 1050 A.D. (?) near Tehachapi; 1500 A.D. (?) near Johannesburg (Searles Valley)

SLIP RATE: between 2 and 11 mm/yr; probably averages about 7 mm per year

INTERVAL BETWEEN MAJOR RUPTURES: between 200 and 3000 years, depending on the fault segment

PROBABLE MAGNITUDES: MW6.8 - 7.6

More at the link below.

• Garlock Fault Info