When do you raise your bet in blackjack?

When to split is up to the player. There are no specific rules about it. However, the most experienced players will tell you to; always split Aces and eights, never split face cards or tens (a 20 is a darned good hand), don't split when the dealer is showing an eight, nine, ten, or Ace. Fours, fives, and sixes usually end up being 14, 15, or 16 (losers). The best time to split any pair is when the dealer is showing a 3, 4, 5, or 6. Deuces (2) are called a dealer's Ace, the dealer will make a hand more often with a deuce showing than any other small card. Remember it is gambling, no guarantees that something will work the same every time, casinos were not built by winners.