It normally takes about 45 minutes for a dryer to dry a full load. If your dryer is taking more than an hour, check these.
Frequently there's an obstruction in the vent duct from the dryer to the outside of the house. For the dryer to heat properly, the duct must be clean and clear of lint or any other substance.
Your dryer's heating element may be partially burned out. If it is, your dryer still heats, but at such a low temperature that it takes three or four times as long to dry the clothes. If the element is partially burned out, replace it.
Your dryer has some internal ductwork. If it gets clogged, your dryer can't dry properly. In most dryers, to get to the internal ductwork to clear the clog, you have to substantially disassemble the dryer. A quick way to check for internal clogs is to remove the lint filter and use a flashlight to inspect the inside of the duct. If it looks clogged and you can't clear it using your vacuum, contact a qualified appliance repair technician.
Although this isn't a common problem, one of the thermostats that controls the temperature in your dryer may break and cause the dryer to heat poorly. If so, you need to replace it. The thermostat is usually a small, round, black device mounted to an oblong steel plate. The plate is mounted to the internal ductwork with two screws.