I received my new glasses today, and whenever I wear them, I get a bit headachy. I asked my doctor why that happened, and he said that my eyes had to adjust to them. How long does it usually take to adjust to the lenses?

It normally takes about 4-7 days to get used to the new glasses.
If after a week and you still don't feel better, then call your doctor/optometrist to schedule a recheck of your eyes.