Hi kildaman No legal time limit as such. This needs to determined by a risk assessment for legionella and any other relevant water borne organism depending on exactly what the CWST is serving and what other risks there may be. The updated HSE ACoP L8 remains your best guide for this. Valid considerations in the RA have always included the amount of debris, the nature of the debris, the speed at which it accumulates, the amount of checking/monitoring you're doing on it and whether legionella etc. remains under control. On the plus side the draft version of HSG274 Part 2 for hot/cold water systems looks like it will include this issue specifically with photos indicating levels that are likely to be considered lower risk and conditions which will likely be higher risk. Not sure if its available yet on the HSE website but should be published by end of March 2014.