There are many factors: Depending on the doctor (they may want to only try 1 week, 1 month, 3 months to see how the med works for you), the medication (controlled substance or not), and of course your insurance. If it is a controlled substance, it can be filled only 1x with the doctor having to write a new script every month, up to 3x in my experience. I generally have to get a refill called in 1-4 times a year (again it, varies) with most of my meds being filled as either 30 days or 90 days (and that depends on the doctor writing the script, insurance, or personal preference). If I am going to be on a med long-term, I ask for 90 days. Most insurances now will cover 90 days for most meds, but there are some meds that will only cover 30 days at a time. Typically, insurance will not allow for a medication to be filled earlier than what it is scheduled for: if it was filled on January 1, for 30 days, it cannot be filled any earlier than 1 week prior to Jan 30 in most cases. I have had insurance not allow a refill until 2 days, it just depends on the medication. For the most part, if traveling, especially abroad, a person can call their insurance to have a script filled earlier than it is time for, to allow the user to have enough of their meds while traveling. But I do not know how they handle controlled substance refills in that situation.

There really are a lot of different answers to your question!

Want to get more complicated? My insurance will cover Gabapentin by mail order or at Walmart Pharmacy with me only having to pay $4. At Walgreens Pharmacy I pay $10. At CVS Pharmacy it will cost me $90. It is all about the insurance companies' pharmacy of choice.
The drug of choice can make the cost different as well. Generic or name brand. Then there are tiers. Example: Gabapentin is not the drug of choice that my insurance likes and it is also a tier III drug. I have to get prior authorizations often from my doctor so insurance will cover some meds because the drug of choice does not work for me/I am allergic to it/I can't take beta blockers. And just to make me more insane, insurance will randomly decide I have to get a new prior auth, and not necessarily like once a year as I would expect.

I receive my medications every 90 days through a prescription assistance program.

I get mine thru the mail and I always have more than I need,so I never run out

Welcome jak try a 3 months supply

The first mistake people seem to make is not paying attention to how much they have left. That seems to be the easiest way to run out of medications.
Plenty of prescriptions for seizures are usually written up to 90 days supply. As mentioned, there are a lot of different Health Care systems that allow you to have them refilled automatically. Otherwise, you can usually contact the doctors office that wrote the script and have it updated or rewritten relatively easy. And of course, the insurance you have has a lot to do with everything in the process.