@Laser9009 although @ShawnC13's response is correct, I just wanted to clarify a couple of things in your question:
"Does a 90 day add-on expire on a prepaid data-plan with roll over data"
So first of all, there's no such thing as a 90 day add-on--as @ShawnC13 explained, the add-ons (other than US Roaming add-ons, which are another story) stay on your account until you have used them up.
But secondly, there's no such thing as a "prepaid data-plan wtih roll over data" currently at Public Mobile. If you're on a 90 day plan with say 6GB of data, any unused plan data from that 6GB disappears at the end of your 90 day cycle and does not roll over into the next one. The add-on data does, the plan data does not. As mentioned, add-on data is only dipped into once your current cycle's plan data has been completely utilized.
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