The short answer is, "until it stops".
Afaik, there aren't any good statistics on averages, and intensities - most attacks go unreported publicly. As many have already mentioned your best, and probably only, bet is to have a candid discussion with your ISP, and rely on their knowledge and experience.
Typically, depending on the tools available and skills of the administrators, you'll be offered with various tradeoffs between site accessibility to the outside world and effectiveness at filtering the DDoS, which will tend to work well enough.
If this sort of attack happens to you frequently, or goes on for more than a few days, it may be worth investing in a better equipped ISP, or, if you're high enough upstream to be responsible for your own DDoS protection, investing into better security tools and people.