If you've got your phone configured to automatically connect to your home wi-fi (as it should be), the remaining data allowance won't go down at all until you're using it away from home.
As a light user of data, you might typically be using 2 GB (= 2000 MB) per month. So, assuming that you're using your phone when it's not connected to your (or to any other) wi-fi network, 150 MB will only last for about two and a quarter days.
Hmmm...that's how long it lasted this week Chris. I was surprised as it has previously lasted upwards of a week...or so I thought. I may have been using unused data that had been carried over when I renewed each month.
I'm just experimenting as the £10 a month plan I had never seemed to get used up.