The 90-day dating rule bases its reasoning on the fact of saying that's how long it takes you to get benefits at a job. So a woman should wait for 90 days because in 90 days most of his crazy ways or whatever behavior will show within that time period and you won't have given him benefits if he really wasn't the man for the job.
Honestly, most people aren't going to do that 90 dating rule. Yes, it would be the safest route but not many are going to choose to do that. If you aren't going to wait until marriage then some type of waiting period or restraint should be practiced.
Some people say 2 weeks, some say until the 5th date, some may do the 90-day dating rule thing, some may say not until marriage, and others may just do it when it feels like the right time. It's a personal choice.