Will 300 MB of data be enough for iPhone 5?

My parents will only pay for 300 MB of data. Obviously I can't watch videos or stream music with this. Will it be reasonably enough to do the following though? Facebook, Siri, probably a movie listings app like Rotten Tomatoes, and looking things up on Google. Plus maybe maps on rare occasions.

The phone only makes a small difference in data usage it's really about what is used. I have always run Android so no clue about Siri, but anything other than streaming and videos you should be fine I would also be wary of pics