It'll probably take around 30 minutes to clean all the fans in computer, maybe less if you have a laptop or tablet, and more if you're using a desktop.
1. Clean the CPU fan, as well as graphics card fan and any other component fans you might have like for RAM modules or other motherboard based chips.
Canned air works great for CPU and component fan cleaning. Keep it upright, make sure the computer is turned off, and do the dust blowing outdoors if possible.
Laptops & Tablets: Your computer may or may not have a CPU fan and likely does not have a fan for other components. If you have trouble figuring out which panel to remove to access the CPU and fan, take a look at your computer's manual online.
Desktops: Your computer will almost certainly have a CPU fan and will likely have a graphics card fan.
2. Clean the power supply fan and any case fans. Canned air works great here too.
Laptops & Tablets: Your computer probably only has one fan and it's blowing out. Avoid blowing the dust directly back into the computer, which could aggravate the fan noise problem in the future. Instead, blow air at the fan at an angle, blowing the dust away from the fan grates.
Desktops: Your computer has a power supply fan and may or may not have inflow and outflow case fans. Blow these fans from the outside and the inside until you don't see any more dust flying out of them.