So as a rule of thumb I would go with 24 lands in a deck.
If the deck is all one color, take out 1 land.
If the majority of your spells are over 3-4 costing cost, add a land or 2 (especially if multi colored and large casting costs).
If you are playing with multiple creatures that produce mana and have multiple mana fixers, then take out 1-2 lands.
Never go below 20 in a single color or 22 in a multi-colored deck.