The click fraud rate varies across industries as Andy Beal mentioned in his post Click Fraud Rate Higher for More Expensive Keywords. Some of my advertisers who exclusively advertise on the Google Search Network have received refunds that exceed 20% of their total ad spend. Other advertisers in a completely different vertical have received refunds of only 5% of their total ad spend.
The best way to determine what your click fraud rate for your specific industry is, is to check your credits on invalid clicks.
You can find this information by looking at your invoice billing statement in your billing account summary. Although Google doesn’t tell you the actual amount of clicks or campaigns that the invalid clicks originated from, you can detect it yourself by comparing your historical data with that months data. Remember to look at patterns on the keyword level as well as the adgroup level.