I went back into tech interviews recently after a 4 year break. It only took two weekends of skimming my old books to get enough up to speed to pass some whiteboard tech interviews, although I don't think I was quite ready for Google level challenges, I'd have probably wanted another week of brushing up for that. If you don't have a few weekends, the quick course is give yourself a graph problem and code it in 30 minutes, critique yourself for 30 minutes, repeat 4 times, go interview. You won't find anything harder on a whiteboard than a graph search, and the practice will be good for you on easier problems.