Typically 1–3 years on gas boilers: flue-gas heat to feedwater adds 3–6 percentage points of boiler efficiency. Condensing economizers do better where return temperatures allow.
From our guide: Heat Recovery Systems for Industry — full context, charts and sources there.
Boilers, kilns, heat exchangers, valves and steam lines lose energy continuously. Inzonex makes patented (UK GB2508992.1) removable modular insulation — snap-fastened covers engineered per temperature tier, not generic off-the-shelf jackets: