I just traded my truck in for a newer one. how long do you need to have it before getting a permit to travel to Mexico?

Other answer:

What? I don't understand your question, and I'm former Homeland Security.

What does your vehicle have to do with getting a "permit" to travel to Mexico?

Are you asking about a "permit" to enter Mexico, to leave the US, something else?

Based on this question – do NOT go to Mexico!

Love big words:
2 days
The fact that you have a new truck is inconsequential to your ability to cross the border into Mexico. A passport and a drug-free vehicle is all you need.
Are you asking if your bank or finance company loan agreement has a restriction on taking the vehicle out of the country? I don't know, you'll have to read it yourself or ask them. If so, you probably can't take it out of the country until you have paid off your loan. The finance company needs to know that they can come get the vehicle if you default on your loan. They can't do that if it's in Mexico.
Rick B:
Huh? What on earth would the age of your truck have to do with traveling to Mexico?