I had an atomizer die in two weeks. Others I've had have lasted 6 weeks and are going strong. They vary. Average is probably a month, but that's based on postings here. If you take care of them, they should last longer. Read threads about the attys for your ecig.
If you are starting with a 510, best to buy a kit (2 batts, 2 attys) and an extra atty and extra batt. Get a PCC case, so you can charge batts on the go. Batts should last around 250 recharges, but that's just what I've read. No where near that many for me yet. Mega batts for 510 are available at Totally Wicked. They last on a charge about 1.8x a regular. So with a few mega batts you tend to not need a PCC case.
The 510 is probably in the long run the most economical PV, as long as you make your own juices. So initial to set yourself up proper with hardware and juices, maybe $150-200. Taking advantage of sales and shopping around are important, but so is having a reliable source, and sometimes there is a trade off there.
2-piece systems with cartomizers tend to be less hassle day to day, but more expensive, and with less flexibility in flavors and strengths. I also got a greensmoke system initially, costing about 2x as much as a full 510 system, and while it is a good system and delivers what it promises, it is not what I generally use on a daily basis.
And like with any tech hobby, the deeper you get into it the more money you spend. I like the science and chemistry of this world, so I spent a fair amount setting up a little mixing lab. You can get bottles, carrying cases, syringes for mixing, special cart filler, pure flavors and large quantities of high-nic juice to mix, extra VG, PG, the list can go on and on. The technology can fit your budget, and most certainly cost far less than analogs, but it can also become fun enough that you want to get more toys, try other models, play with different juices. Its up to you and your needs, home environment, and budget.
Have fun!