Only the 200 MB and 1 GB data add-ons do not expire.
The data that you are seeing here would be the 6 GB of data that you have included as part of your plan. You've used 1740 MB (so 1.74 GB) of data from your plan so far.
If you are on a 30-day plan, this will reset every month, and with a 90-day plan that 6 GB would be for the entire period so it would be best to split it out over 3 months (6 GB / 3 = 2 GB available per month)
The best way to look at your data usage is from self-serve as it will show you an accurate number based on Public Mobile's usage tracking. If you're using an Android, then the built-in data usage tends to work fairly well for the most part, just make sure that the data cycle is set to reset on the correct date (your renewal date, which can be found in self-serve on the main page).
