Two Turn Gameplan Rule
It differs from deck to deck, but the gist of the rule is: Do you have a plan about what you're going to do in the first two turns? This usually means that you have two lands and a couple plays. You should also imagine the position you will be in on turn three and see if it's acceptable.
Of course, this varies from format to format. Some formats allow you to do less in the early turns, whereas others promote lots of plays in the early turns.
Let's run some numbers real quick. In a 24 land deck, you should draw an average of 2.8 lands per seven card hand.
2.4 land average for six card hand.
2.0 land average for five card hand.
1.6 land average for four card hand.
1.2 land average for three card hand.
0.8 land average for two card hand.
0.4 land average for one card hand.
0 land average for zero card hand.
If you have two lands in your opening seven, and you may want to mulligan, keep in mind that you are more likely to have two lands than three lands in a six card hand.
If you follow the two turn gameplan rule, and keep this hand (considering that the rest of the hand fits the rule), then this is what should happen.
Turn One: 40% Chance of drawing at least one land
Turn Two: 80% Chance of drawing at least one land
Turn Three: 120% Chance of drawing at least one land
If you're on the draw, you are almost guaranteed to draw a land on turn three.
On the play, it's sometimes safer to mulligan a two land hand, depending on how good the rest of the hand is. By turn three on the play, you'll only have had an 80% chance to draw your third land.
Some decks need less or more lands to function. That will be the topic for the next When to Mulligan. I hope you enjoyed it!