Photocatalytic degradation of Methylene blue (MB)

I have run some experiments to test the photocatalytic activity of my TiO2 nanoparticles. I found that for a small load of TiO2 there is no change in the MB conc. up to 3hrs under a UV treatment, and by increasing the amount of TiO2, this period got smaller. I am not sure what I should consider this period as, 3hrs?
Should I start my curve to show the photocatalytic effect after this period?