YOU DEFINE "LONG TERM": whether a LTSE has been sampled meticulously for >100 years such as some of Rothamsted's studies, or a study has just been sampled for the first time but has ambitions to last for 200 years (e.g., the Vermont-USA LT Soil Monitoring Study).
The users of the LTSE database can sort by duration of study, so YOU define "long term".