How does brewing time and brewing temperature affect the flavonoid concentration of green tea?
You might want to have a look at

J. Peterson et al., Major flavonoids in dry tea, Journal of Food Composition and Analysis, 2005, 18, 487--501, http://dx.doi.org/10.1016/j.jfca.2004.05.006

and the references therein, e.g.

Christiane Lakenbrink et al., Flavonoids and Other Polyphenols in Consumer Brews of Tea and Other Caffeinated Beverages, J. Agric. Food Chem., 2000, 48*, 2848--2852, http://dx.doi.org/10.1021/jf9908042

From the abstract of the latter:


  [...] At brew times up to 2 min the composition of the brew solids was for each product practically independent of brew time, with flavonoids again comprising the major proportion (86−88%) of the total phenolics. [...]


Longer extraction times (> 10 min) or continuous (Soxhlet) extraction lead to partial decomposition of the flavonoids, most likely due to phenolic oxidation, cf.

Won Jo Cheong et al., Determination of Catechin Compounds in Korean Green Tea Infusions under Various Extraction Conditions by High Performance Liquid Chromatography, Bull. Korean. Chem. Soc., 2005, 26, 747--754, http://dx.doi.org/10.5012/bkcs.2005.26.5.747

I hope that helps a bit.