Day 1-28 — the discount window
From the date the FCN is served, you have 28 days to pay €40 (parking FCNs) or submit a written representation to the issuing authority. The day of service counts as day 0 — count from the day after.
Day 29-56 — the doubled fine
On day 29 the fine increases to €60. You can still pay or maintain a challenge during this window but the discount is gone unless your representation is upheld.
Day 57+ — summons risk
After day 56 the issuing authority can refer the matter to the Director of Public Prosecutions, who issues a District Court summons. The court can impose the original fine plus court fees, and (for Garda FCNs) penalty points.