Each state defines their eligibility requirements differently. In some states, you only need a 20% delay to qualify for services, in others it is up to 50% delay though! The percent is based on age equivalence, so that would mean that at 24 months, your child would be saying less than what would be expected at 12 months for a 50% delay - which is a huge delay! Other states base their eligibility on other measures, such as standard deviation from the norm.
If you are in Florida, then they require a delay by 2 standard deviations from the norm if delayed in one area (ie speech only) or a delay of 1.5 SD if delayed in more than one area (ie speech and PT or OT). For a 2 SD delay, this is equivalent to a score of 70 on a standard score test where 100 is average and SD is 15 (the way a standard IQ test is scored). It is also equivalent to the 2nd percentile rank.
To see the requirements for different states, see here:
Sign In
Where I live, in NC, they require 30% delay in 1 area, or 25% delay in more than one area. It varies a lot by state.