With my gf, we went
1 week (bought her a rose

)
1 month,
2 month,
6 month,
1 year,
then every year
then a really big one at 5 years.
Keep in mind it we started in high school.
It all comes down to the couple. The people in the relationship have different backgrounds and different wants than others. They may have had an issue with long term relationships so they wants to celebrate the anniversaries.