What Dominion kingdom-card setups tend to lead to the longest games?
Basically, any card that denies opponents their resources (i.e. cards in hand) or pollutes their decks (e.g. curses) makes for a longer game because it is more difficult to buy victory point cards for lack of money or just forces an interaction that requires a decision in your opponent (i.e. this increases the time, not necessarily number of hands).

Examples:

Militia - reduces all opponents to three cards, also increases time by a bit.

Bureaucrat - forces all opponents to put one of their victory point cards on top of their deck.

Spy - cycles through good cards in opponent decks, leaving until only a victory point cards or useless card is left on top, also just increases time.

Thief - trash or steal opponent treasure cards, also just increased time.

Witch - adds useless cards to opponent decks.