"which usually is the case for ..." or "which is usually the case for ..."
In AmE, I would say ..., which is usually the case is used more often than the first example, as it flows more readily.
Gas furnaces produce less particulate matter than oil furnaces when both are maintained infrequently, which is usually the case for residential ... (NYT)
...which is usually the case for the major traded currencies... (WSJ)
His ball striking is very crisp which is usually the case for him when he's... (USA Today)