What is the converted mana cost of land cards?
It's 0†, so Sun Titan can bring them back.
This is also noted in the rulings below the card in Gatherer:

If a card in your graveyard has no mana symbols in its upper right corner (because it’s a land card, for example), its converted mana cost is 0.

†: with the exception of transformed lands, as noted in @NeilMeyer's answer. This situation doesn't happen in the graveyard, though.