The symptom is a clock, not a smell
Ask people how they knew their dryer duct was blocked and most of them describe the day it stopped working. The months before that, when a load went from 45 minutes to 50 to 58, went past without anyone counting, because a dryer that takes an hour still puts dry clothes on the bed and nobody times it.
With the numbers in this form — 62 minutes now against 45 minutes recorded when the run was clear, five loads a week all year, a 5 kW machine — the seventeen extra minutes come to 73.7 hours of extra drum time, 368 kWh, and 58.93 dollars a year at sixteen cents. That is 23 cents a load, which is exactly why nobody notices it a load at a time.
The baseline is the whole thing
Everything on this page hangs on the second field, and it is the one figure that has to be yours. There is no standard cycle time. A machine takes as long as it takes on your load size, your fabric, your extraction, your laundry room and your duct, and comparing today against a number from anywhere else compares two different situations.
Which means the useful move is to write down a cycle time the day a duct is cleaned or a machine is installed, on a load you would call typical, and keep it. Without that, this page can still tell you what a hypothetical seventeen minutes would cost, but it cannot tell you whether you have them.
Cleaning does not always pay on energy
Run the defaults with a 150 dollar quote and the energy alone covers it in about two and a half years. Then look at the interval line: if the run drifts back to where it is now every 24 months, the cleaning is 75 dollars a year against 58.93 dollars of energy, and on money alone the maintenance loses by about sixteen dollars a year.
That result is honest and it is also incomplete, which is worth saying plainly. Lint accumulating in a hot duct is a fire risk, a restricted run pushes moisture back into the house, and on a gas machine it interferes with getting combustion products outside. None of that is arithmetic and none of it appears above. The page prices the energy because the energy is the part that can be priced.
Where the number is soft
The calculation charges every extra minute at the running power of the machine. A dryer that heats continuously through the cycle is charged correctly; one that senses dryness and cycles the element on and off is charged too much, sometimes by a lot, which is what the duty share field exists to correct. If you have no idea what your machine does, read the yearly figure as a ceiling rather than an estimate.
The other soft edge is attribution. A cycle can get longer for half a dozen reasons that have nothing to do with the duct — a tired element, a washer leaving loads wetter than it used to, a lint screen that never gets cleared, a colder room in winter. This page prices minutes. What is causing them is a different question and one for whoever works on the machine.
Questions people ask
How much does a longer dryer cycle actually cost?
At the defaults, seventeen extra minutes on five loads a week for a year is 73.7 hours of extra running, 368 kWh and 58.93 dollars at sixteen cents a kilowatt-hour. Per load it is 23 cents. The figure scales straight with the machine power, so a 3 kW machine on the same schedule costs about 35 dollars and a 6 kW one about 71.
I never wrote down a baseline cycle time. What now?
Then this page cannot tell you what you have, only what a given difference would cost. There is no standard time to fall back on and inventing one would just move the guess into the second field. The practical move is to record a time on a typical load now, keep it as the reference from here on, and treat today as the new baseline whether or not the duct has ever been cleaned.
Does a longer cycle mean the vent is blocked?
Not on its own. A heating element that has partly failed, a worn drum seal, a lint screen nobody clears, a washer leaving loads wetter than it used to, and a cold laundry room all lengthen a cycle. The clock tells you something changed and roughly what it is worth. Which of those it is gets settled by somebody who works on the machine, not by arithmetic.
Is duct cleaning worth the money?
On energy alone, at the defaults and a two-year drift, no — 75 dollars a year of cleaning against 59 dollars of energy. The page will tell you which way it lands at your quote and your interval. It stops there deliberately, because the other reasons people clean ducts are lint in a hot duct, moisture pushed back into the house, and combustion products on a gas machine, and none of those are things a calculator gets to weigh.
Why does the gas option ask for electricity as well?
Because a gas dryer still turns the drum and runs the blower on electricity for the whole cycle, so extra minutes cost both fuels. The burner input goes in as BTU per hour off the data plate and the motor as a kilowatt figure, and the page prices the two separately so you can see which one the extra minutes are actually running up.