Deciding when to donk bet involves considering various factors, including the board texture, your hand strength, the aggression level of your opponent, and the table dynamics. You should also think about the range of hands your opponent is likely to continue with and how a donk bet will affect their decision-making. A donk bet can be an effective tool for building the pot with strong hands, protecting your hand from free cards, and disguising the strength of your hand, but it requires careful consideration of the specific situation.