@Kiicki you will probably need to change your coil every 1-2 weeks, but it is going to depend on a lot of factors, like what tank you get, what kind of juice you vape, how often you vape, what your device settings are, etc... there are too many variables to accurately predict how long a coil will last. That is something you will figure out as you go along.
For just starting out, you should get a couple of extra packs of coils when you buy your mod, that will give you 10-12 coils (2x 5 packs, plus 2 with the tank usually), which should be enough to start getting a feel for how long they last. If you get 2 weeks per coil, and have one tank, then 12 coils should last 24 weeks, almost half a year for only a little bit of money. If you can afford to buy the coils, that will always be easiest and most convenient. Your mileage may vary with coil life, when I was using tanks with commercial coils, I got 3-4 weeks out of some, but only 2-3 days out of others.
Also, you have to buy some coils, at least in the beginning, because you need the "shell" to put your own coils in. You will need some used coils to rebuild, so save them when you replace them, don't throw them away. You can also try "cleaning" the coils, but it may not work. The best way I have found to clean them is to soak them in Pure Grain Alcohol over night, then rinse them under running hot water until the water is clear, then let them dry for about 24 hours. If you try this, then they will need to be primed again before you use them.
The process for rebuilding a commercial coil is different for each coil design. some are easier than others, but all will require practice and patience until you get the hang of it. The easiest way to find the information is to search the internet for the specific instructions for your tank model. There is a ton of info available for almost every mass produced tank on the market.