How many times will my Data Roll Over

With Data Roll Over, iD states that my unused data for last month will roll over to the next month.
Now will this only work for immediate prior months or work for all prior months
Essentially if I had 2 GB in Month 1, used 1GB, thus having 1GB rolled over on to the next month
My new data for the Month 2 would then be 3GB (I understand this, fairly simple)
Now if I used only 1GB again in Month 2, then would:
Scenario 1: 2GB (Month 1 + Month 2) be rolled over to Month 3.
or
Scenario 2: 1GB (Month 2 only) be rolled over to Month 3.


If Scenario 1, I would like to know what my stacked data limit is or if there are a limit to how many months worth of data can be rolled over.

If Scenario 2, I would like to know if my data would roll over if I hadn't touched it that month.