Same here. I tried on the starter plan but had to upgrade. According to my account I have made 802 searches since January 4th. So 17.4 searches a day on average. This means that for a 31 day month I am looking at 620 searches.
I am also a heavy user of bookmarks and browser history. So I don't rely on search to open specific sites (like searching for "facebook" which is one of Google's most popular queries). So someone who is in the habit of using search for direct navigation is probably going to be a good chunk higher.
That being said I work on the computer and do a fair number of searches for my job. So I can believe that a light user is pretty comfortable at 300 searches a month. But moderate searches or people who use the search engine for navigation will need the unlimited plan.