On average, trophy-taker's works out to be a net increase in assignment length. If however you do fewer kills than assigned you will receive fewer slayer points for completing the task. When the perk doubles kill count, it prompts the message "Trophy-taker's perk: this kill has added double to your Slayer task tally." When a kill does not count, it prompts, "Trophy-taker's perk: this kill did not contribute to your Slayer task tally."