My first coil burned after 4 days, the second after 2 days, and the last one even didn't soak up and I burned it instantly.
The instantly-burned coil leads me to ask if you're priming your coils. Priming means making sure juice is soaked in before you fire it up the first time. The generally recommended procedure is to put a few drops of ejuice on your coil - a couple drops from the top, then look on the sides for holes where you see cotton and drop some juice there, let it soak in, and add a few more drops. Then fill the tank and let it sit for 5 - 10 minutes. You can also close the airflow and take a few puffs without firing the mod. There's a lazy version too - if you don't want to do all that, you can just fill the tank and wait longer before you use it.
The point of all this is to make sure the wick is saturated before you start heating the coil. If there's no juice at the coil, you get instant burn. If there's not enough juice, you can stress the coil and shorten its life.
Some of the other things that affect coil life are how much you vape (the chain-vaping) and the juice you're using. A juice with higher PG will wick faster, so that might help. Increasing the PG will also increase the throat hit and some people are sensitive to it, but others don't mind. Also, sweeter and darker juices tend to leave more residue and gunk up the coil faster than lighter and less-sweet juices.
There are many factors to coil life, so I wouldn't expect you to get the same results as your friend. One to two weeks is pretty normal for a coil, but some will get more and others less.