Will my iPhone carrier let me know when I've used up my monthly 4G?

I have an iPhone 5, and my carrier is AT&T. My sister has told me that the day I pay my phone bill, I can use 4G until the 30 days are up and I have to wait until my next plan to use 4G again. I'm really anxious because I can't tell how much data I'm using and when I should stop before I'm charged for going over the limit. I've gotten a text from AT&T saying I had used 65% of my data, so will they send me one for when I use all of it (100%)?

They will. Basically every phone service will warn you somewhat close to the end of your 4G data and when it is used up completely. Now I don't use AT&T since it doesn't exist in my country but, it will text you, and it may close down your 4G connection, if not, you will have to shut it down when it tells you, so you don't take extra charges!