Hey Bro, It can not last for long when you use it for downloading and streaming.
One thing too that makes internet very expensive on iphone is that it uses high definition so when you open a webpage, it opens in HD mode which kill the data.
Also try to remove the automatic update from your iphone.
I use iphone 5 and I always buy 1Gb of data. I just use it for what's App and small small things download too but it reach a week.
Yours is very small but If you dont make big downloading and streaming with it, I Expert it to reach or exceed a week.