  We derive the minimum observing time scales to detect a blazar at a given
flux level with the LAT on GLAST in the scanning and pointing modes. Based upon
Phase 1 observations with EGRET, we predict the GLAST detection rate of blazar
flares at different flux levels. With some uncertainty given the poor
statistics of bright blazars, we predict that a blazar flare with integral flux
>~ 200e-8 ph(> 100 MeV) cm^{-2} s^{-1}, which are the best candidates for
Target of Opportunity pointings and extensive temporal and spectral studies,
should occur every few days.
