Regulations don't specify an actual time frame for filtration, however it is recommended that the samples be filtered in the field so that they may be immediately preserved.
This increases the stability of the target parameters and ensuring representative results.
If filtration is not possible, samples should be collected without preservation and submitted to the laboratory as quickly as possible, preferably same day. This allows the lab to filter and preserve the samples within 24 to 48 hours from receipt (please note that unfiltered and unpreserved samples submitted during weekend hours may take longer to be processed).