Applies to 1/2-5/10 NL:

You play more than 15 hands at one table without 3-betting, if you get 4-bet by a regular when you 3-bet for the first time he is always bluffing or semibluffing (4-betting a weak hand he plans to stack off with).

examples:

Full Tilt No-Limit Hold'em, $6 BB (3 handed) Full-Tilt Converter Tool from FlopTurnRiver.com (Format: FlopTurnRiver)

BB ($252.75)
Button ($1339.90)
Hero ($597)

Preflop: Hero is SB with 6, 7.
Button raises to $21, Hero raises to $75, 1 fold, Button raises to $189, Hero raises to $672 (All-In), Button folds.

Final Pot: $384

PokerStars No-Limit Hold'em, $6 BB (6 handed) Poker-Stars Converter Tool from FlopTurnRiver.com (Format: FlopTurnRiver)

Hero ($1376.75)
BB ($634)
UTG ($707.80)
MP ($723.45)
CO ($625.95)
Button ($418)

Preflop: Hero is SB with T, A.
2 folds, CO raises to $21, 1 fold, Hero raises to $72, 1 fold, CO raises to $198, Hero raises to $1376.75, CO calls $427.95 (All-In).

Flop: ($1257.90) 7, 4, 6 (2 players, 1 all-in)

Turn: ($1257.90) 5 (2 players, 1 all-in)

River: ($1257.90) 7 (2 players, 1 all-in)

Final Pot: $1257.90

Results in white below:
Hero has Ts Ac (one pair, sevens).
CO has 9d 9h (two pair, nines and sevens).
Outcome: CO wins $1257.90.