What happens when you win
A new assessed value, a lower bill next cycle, and a reduction that usually persists.
You receive a written decision setting a new assessed value.
The bill
Because Illinois taxes are paid in arrears, the reduction shows up on the bill for that assessment year — generally the one you receive the following year. If you have already paid on the higher figure through PTAB, you receive a refund.
How long it lasts
The reduced value normally carries forward until the next general reassessment of your township. That is what makes a modest annual saving worth real effort: it is the annual figure multiplied by the years remaining in the cycle.
What it does not do
It does not reduce what your taxing bodies collect — the levy is fixed and the rate adjusts. Your share falls; everyone else's rises fractionally.
What to do next
Check the following year's notice to confirm the new value carried forward. It does not always. If the old figure reappears without a reassessment, call the assessor — that is a correction, not a new appeal.