For the modern male the theory translates as follows: No matter how hot or unhot a woman is, eventually you will want to sleep with her. The time it takes for this process to occur is the chick's "Mermaid Clock." It starts the first time you lay eyes on a chick* and stops ticking the instant you want to get your jam on.
As an example, if you met Scarlett Johansson, her Mermaid Clock would be .00000001 seconds. She's hot and you would immediately want to inspect those two grapefruits she's strutting around with. Conversely, if you met a less attractive woman, her clock could last anywhere between a couple of hours (e.g. she's nearing 30) to a couple of years (e.g. she's nearing 35).
Once a girl turns into a mermaid she can again turn into a manatee if she throws up in front of you or if she gets pregnant.