These products usually start working instantly, killing existing fleas in as little as 12 hours. (pawster.com)
Thanks to this new chemistry, Advantage ® begins working almost immediately, killing 98 - 100 % of existing fleas on cats and dogs in just twenty - four hours. (safarivet.com)
It kills up to 100 % of existing fleas in less than 24 hours. (1800petmeds.com)