A typical Web app is attacked once every three days and some are targeted as many as 2,700 times in a given year. Imperva monitored 50 Web applications for six months, looking at the kinds of attacks each one endured and pulling out trends. The average attack that Imperva observed lasted a little less than eight minutes and the longest went on for about 80 minutes. Some of the applications are under attack as often as 292 days per year. The report found that SQL injection was the most common attack type.
Source: https://threatpost.com/average-web-app-attacked-every-three-days-080812/76891/