Depends on the wording of the rule, by default a model can only make a pile-in move when it is within 3" of an enemy model (or if it made a charge move in the same turn) no matter how long it's pile-in movement distance might be.
However a lot (almost all) of the units/rules which allow for a longer pile-in move will also say something like, 'This unit may be selected to make a pile-in move when within 6" of an enemy unit, and may move up to 6" when it does so,' or similar, which would overrule the default 3" rule. So it does depend on the specific unit in question strictly speaking, but almost all of them have an exception of this kind allowing them to pile in at 6".
Lucentia's post in Pile in 6" was marked as the answer