We never really get to know what happens between flashback jumps; however based on last season's timeline Oliver left Lian Yu for Russia sometime around January 2011. So he's been there a good 9-10 months on his own, learning Russian and fighting underground getting himself into shape
In case anyone is wondering how I came up with my between season estimate, is last season's estimate puts those flashbacks at Dec 2010/Jan 2011ish at its end. Given the hair growth its reasonable that Oliver's been in Russia 9-10 months; human hair grows at about a half inch a month. In comparison to how we left him, looks like 4-5" growth to me.