How many times has NAIP been acquired for California? 
According to DFG, we have:

• NAIP 2014 aerial imagery, 1 m, 4 variations (natural color, 4-band, CIR/false color, NDVI)
• NAIP 2012 aerial imagery, 1 m, 4 variations (natural color, 4-band, CIR/false color, NDVI) 
• NAIP 2010 aerial imagery, 1 m, 4 variations (natural color, 4-band, CIR/false color, NDVI) 
• NAIP 2009 aerial imagery, 1 m, 4 variations (natural color, 4-band, CIR/false color, NDVI) 
• NAIP 2005 aerial imagery, 1 m (natural color)

I was not aware the flight schedule was this frequent. 

Still, I can't find a definitive information source that helps.