It depends on how strong you want the raspberry flavor to be. For my blueberry wheat I use 1lb of frozen blueberries, leave them in for about 5 days then rack to a tertiary for a few more days to let the remaining blueberry debris settle out. This gives a nice light blueberry taste, not overpowering at all. If you want a stronger berry taste use more berries or just leave them in longer.
To prep my berries I basically pasteurize them on the stovetop. Just put them in a sanitized sauce pan with a little water and hold them around 170-180* for about 15 minutes, let them cool then rack onto them in the secondary
Knucklehead Brewery, Est. 2007
Always do sober what you do drunk. That will teach you to keep your mouth shut. -Ernest Hemingway