Glass fibre reinforced polyester (GRP) has been used in sewer pipes for decades, and has demonstrated excellent performance in concentrated sewerage. It is well documented that sulphuric acid – developed through the action of bacteria such as thiobaccillus concretivorus and thiobaccillus ferrooxidans – may aggressively attack any pipe material. Over the years GRP pipe samples have been subjected to an array of different tests to establish their lasting performance. One of the greater challenges when conducting such tests is the time factor. To get any meaningful results, samples have to be exposed to the chemical environment at elevated strains or temperatures for a considerable time. To determine the long-term properties the data has to be analysed statistically and extrapolated into the unknown to predict a limiting strain for use in pipe design. The acid test method proving the long-term performance of GRP pipes was first standardized by ASTM in 1978, and has been in use ever since. GRP pipe producers have collected a remarkable amount of test results based on this method. The method calls for at least 10 000 hours of testing with at least 18 samples, but as the test is relatively simple to conduct – and does not require much space – a great number of samples have been left exposed to acid for much longer time periods. This paper addresses the results of such longer-term tests and how they relate to the standardized method and regression analysis. Over 600 data-points are analysed, with times-to-failure ranging from a few hours up to 28 years. Analysis of the data demonstrates an interesting bi-linear behaviour, rather than the straight line regression predicted by the shorter and smaller database. The results suggests that the standardized method is indeed quite conservative and that with this additional information the safety margins are shown to be higher than expected and extrapolation even up to 150 years can be realized. Inspection and testing of samples taken from sewer pipes that have been in service for almost 24 years – and found to be in impeccable condition – further corroborate these findings.