My neurologist put me on propranolol (10mg) for migraine prevention for one week, then had me increase the dosage the next week to 20mg. just curious as to how long it will take for it to help with migraine prevention?

Preventative : The dose depends on severity and frequency of your migraines. Improvement may require multiple dose increases. Good Luck

Few weeks: It can take a few weeks for propranolol to improve your migraines. Some data suggests it can take up to 6 weeks. Your neurologist may be increasing your dosage a little bit every week until it starts helping your migraines. It will be worth communicating with your neurologist if you continue to experience migraines after a few weeks.