The short answer is when there is a 1/4" of soot on the surfaces. This was the figure arrived at in the Manual for Certified Chimney Sweep Program of the Chimney Safety Institute; it provides a good rule of thumb. It has been shown in tests that 1/8" (3mm) of soot covering a back-boiler can reduce it's efficiency by 25%; meaning you will have to burn 25% more fuel to get the same amount of hot water, food for thought. Given that most back-boilers work at around the 30% efficiency it makes sense to get the most out of your fuel as you can, doesn't it?
After a chimney fire: many people think that because the chimney has caught fire it does not need sweeping; however, there is a very good chance that there is burnt soot stuck to the surface of your chimney. This 'clinker' as it is known has a huge surface area and attracts soot like a magnet. Therefore, if the chimney is not swept clear after a fire it will quite possibly be a much shorter time until the next fire. (see here)
Finally, Stove flues need to be clear to ensure safe ejection of Carbon Monoxide gas. Carbon Monoxide is the 'silent killer'. Any blockage, including spider-webs can and have caused noxious gases (including Carbon Monoxide) to get into a room when lighting a stove. (see here)
You should always have your chimney swept before the winter.
This is a conversation that we will have with you following the sweep; however, HETAS and the SFA agree that depends on what you burn and how much you use the fire/stove.
Smokeless fuel - Once a year
Domestic Bituminous Coal - Every six months (when in use)
Seasoned wood - Every three months (when in use)
Green/Wet/Unseasoned wood - not advisable to burn this sort of wood, it will lead to tar and creosote build-up which in turn will lead to a either chimney fire, blockage or corrosion of steel flues.
Oil appliance - once a year + checking by OFTEC technician
Gas appliance - once a year + checking by a Gas Safe technician
Doing a proper job...
The following images show:
1) an unswept chimney flue
2) Chimney flue after four passes of rotary sweeping through a sheet and a vigorous hand-brushing
3) Chimney flue after using camera to inspect the gather and then subsequent sweeping to target stubborn/hard to get deposits which involved sweeping the gather without the sheet closing off the entire fireplace opening.
The only way to really get this sort of clarity is the use of digital camera. Beware of sweeps who may just push the brush up and down against the back wall or wave the vacuum nozzle around in the gather. Most chimney fires start in the first two metres of the flue!