how long should a 16gb phone last?

I recently got the Iphone 5s and I like it alot but I'm learning that in the past 3 days that I've had it I've used 2.2gb. My phone is supposed to be a 16gb but only shows 12.9 capacity which I don't understand either. I just want to make sure my phone is going to last me some time or do I need to upgrade to a larger capactiy, some help please.