Most Colorado cars want a full detail twice a year and a wash every one to two weeks, with the washing mattering far more between November and April. That is the short answer, and the rest of this explains when to deviate from it.

The baseline for a normal daily driver

Two full details a year and a wash every one to two weeks keeps almost any daily driver in good order. One detail in spring to strip off the winter, one in autumn to go into it protected.

A full detail is $249 and covers inside and out. It is the standard booking for a reason.

Why winter is the half that counts

Winter is when the damage happens, because magnesium chloride de-icer stays wet on the car and keeps working long after the road has dried. A car washed fortnightly through winter and left alone all summer will be in better condition than the reverse.

Between November and April:

Rinse every one to two weeks, and always within a few days of a treated storm
Flush the undercarriage, which is where the corrosion actually starts
Rinse before wiping, because dry salt crystals are abrasive

When you should go more often

Four situations justify more:

Parked outside all year. Ultraviolet at 5,280 feet is roughly a quarter stronger than at sea level, and it is the main cause of clear coat breakdown. Outdoor parking moves you to three details a year.
Dirt roads or active construction. Fine dust is abrasive and bonds to paint. Common on the north and east edges of the metro.
Dogs, kids or a work vehicle. Interiors, not paint, drive this one. An interior detail is $169 on its own.
You are selling within six months. Do it before the photographs, not after the first viewing.

When you can go less often

Two situations justify less: a garaged car that does low mileage, and a coated car.

A ceramic coated car genuinely needs less. The coating does not remove the need to wash, but it makes washing quick and it stops most of what lands on the paint from bonding to it. One full detail a year plus regular washing is usually enough.

Where a maintenance plan fits

A plan makes sense when the problem is remembering, not money. Most people know they should wash the car fortnightly in February. Almost nobody does.

Monthly maintenance is $130, billed every four weeks. It exists because the interval is what protects a car, and an interval only holds if somebody else is keeping it.

The one thing not to do

Do not use an automatic brush wash to keep up the interval. Those brushes are the single most reliable way to put swirl marks into a clear coat, and swirl removal is paint correction at $399. A fortnightly touchless rinse is better for the paint than a fortnightly brush wash, by a wide margin.