When I was younger, say in my late teens, I would have six week flings or a year + long relationships. There was rarely anything in between these two states of being.
In six weeks, I could meet someone, fall in love, plan for the future, wake up one day (usually day 42) and be out of love, the feeling of love, lust or euphoria gone. I started to hold back in the first few weeks to see if what I was feeling was actually real. I didn't want to say anything that might lead someone on or be subject to change.
A year +
If someone lasted past week six, they were a keeper. The six week rule started as an in-joke between my female friends and me, it wasn't a rule as such, just a very handy benchmark.
When I reached my twenties I was aware of the giddiness that I could go through early on and started to quell that side of me, sensibly.
This had good and bad points. I could be colder and more hesitant, I was less spontaneous and romantic, but when the six weeks courting was up, I was then in a better place to see the person without the daze of early excitement.
There are different times of year where this feeling can be exacerbated, Christmas and birthdays can be magical enough to break down even concrete supported boundaries.
Nowadays I still have those boundaries in place, yet can smell the coffee so much earlier one rarely needs six weeks.