"I have always heard that one should wait as long as possible before having it professionally cleaned..."
Here's the deal.
When a carpet cleaning company uses soap and water to clean your carpets, it is almost impossible to rinse all the soap out of the carpet.
Allow me to insert here that soap, being soap, collects dirt. That's what soap does for a living. It sits there and collects dirt.
Now back to your carpet. The soap that doesn't get rinsed out of you carpet is still soap. It is still collecting dirt. It doesn't know that it is supposed to stop.
That's why carpeting gets dirty faster after it's been cleaned using soap. The soap is sitting there collecting dirt.
Therefore the wives tale was always to wait as long as possible before cleaning new carpeting.
That being said, not every good carpet cleaner uses soap and water. Some use a dry product and I know of one company that uses a special, proprietary water product that works very, very well cleaning your carpet without soap.
If you go in that direction, then you need not be afraid to have that new carpet cleaned.
One note of interest…
I am told by those that know that you should follow this formula for how often you should vacuum your carpet (new or not).
Count one for each family member and each indoor pet. The total is how often you should vacuum your carpet each week for it to be considered clean.
Example: you are married with two kids and a lap dog. That makes 4 people and 1 indoor pet which makes 5. If this is you, you should be vacuuming your carpets 5 times per week.