Probably the best way to answer this question is to look at tolerable upper limit estimates for DHA and EPA that have been set by expert organisations. This page states that the US FDA recommends less than 3,000 mg per day of combined EPA, DPA and DHA intake (apparently it is not possibly to separate the three here). This typically would mean that no adverse affects have been observed below that level.
Probably the best way to answer this question is to look at tolerable upper limit estimates for DHA and EPA that have been set by expert organisations. This page states that the US FDA recommends less than 3,000 mg per day of combined EPA, DPA and DHA intake (apparently it is not possibly to separate the three here). This typically would mean that no adverse affects have been observed below that level.