A dryer that used to finish a load in forty minutes and now needs an hour and a half hasn't necessarily developed a major fault. In our experience, the actual heating element is the least likely cause on this list, even though it's usually the first thing people suspect.
Restricted vent airflow
This is, by a wide margin, the most common cause. A dryer works by pushing hot, moist air out through the vent — if that path is even partially blocked by lint, the machine has to run longer to reach the same dryness, and in bad cases never quite gets there. Our full guide on signs your vent is overdue for cleaning covers how to tell.
An overloaded drum
Dryers need room for air to circulate around the clothes. A drum packed too full — often bedding or towels, which absorb more water than a mixed laundry load — simply can't dry as efficiently no matter how well the machine is working. Splitting a large load in two sometimes solves the problem entirely.
A failing moisture sensor
Many modern dryers use a sensor cycle rather than a fixed timer, ending the cycle once clothes reach a target dryness. If that sensor has picked up a layer of fabric softener residue, it can read clothes as wetter than they actually are, extending the cycle unnecessarily. Wiping the sensor strips (usually two small metal strips inside the drum) with rubbing alcohol occasionally solves this without any parts needed.
A genuinely weakening heating element
When it is the element, drying time tends to increase gradually over weeks or months as the element loses efficiency, rather than dropping off suddenly. If cycles are getting steadily longer and airflow and load size are both ruled out, this becomes the more likely explanation.
Low household voltage on electric dryers
Less common, but worth a mention — an electric dryer sharing a circuit with heavy appliances can occasionally underperform if it's not getting full voltage. This is more of an electrical question than an appliance one, and worth a mention if the dryer is on an older or shared circuit.
Quick way to narrow it down: if a single, reasonably sized load still takes noticeably longer than it used to, check the vent first. If a properly sized load dries fine but only large loads run long, that's more likely just a load-size issue than a fault at all.