three to six weeks

I'm guessing it's also a different definition of fasting than most expect. When you fast before a surgery, you eat literally nothing. When you fast in Lent, you abstain from certain foods, but not all foods. When you fast in Ramadan, you completely fast during the day, but eat after sundown.

We don't know the specifics, nor the specific amount of time, and I doubt that actually matters anyway. The point of the story is that Jesus was tempted while at his lowest point and still remained faithful. That's it.