How many years should a parge coat realistically last?

Roughly 10 to 25 years. Where a coat lands in that range depends on how well the wall was prepared, how much water reaches it and how many freeze-thaw cycles it goes through. Coats on well-prepared walls with good drainage last toward the top of the range. Coats applied over dust, paint or failing material, or on walls where soil and downspouts keep the base wet, can fail within a few years. Most early failures trace back to poor preparation or trapped water rather than the coating wearing out. You can extend a coat’s life by keeping soil sloped away from the house, directing downspouts away from the wall and patching small damage before water gets behind it. If your parging is more than 15 years old, a free parging inspection tells you how much life it has left.