In my opinion if properly maintained a colorbond fence will outlast a timber fence by 5-10 years. Colorbond fences are made from steel so the paint will fade and the steel eventually rust but not so quickly as treated pine or even hardwood posts will rot through enough to lose stability.
A modification growing in popularity to increase the life of the timber fence is to use Galvanised steel posts. Galvanised Steel posts may eventually rust but the steel is thicker than colorbond steel so the posts will outlast colorbond posts however the timber will still generally warp, fade and rot therefore still living less time than a colorbond fence however if the fence is maintained correctly when the timber has seen its last days you can simply cut the timber frame away and re-use the same steel posts for your next fence.
This will reduce the cost of your next fence substantially because most of the labour in fence building is the setting of the posts which won’t be necessary. The price of a standard 1.8m high paling fence is not increased using steel posts as light gauge steel posts can be utilised for the same cost as timber posts.
If you wish to upgrade to a higher fence or lapped palings then this will increase the cost of the steel posts as heavier gauge steel will be required.Steel posts will increase the life of your timber fence however many people don’t like the look of steel posts and wish their entire fence to be made from timber therefore opting for timber posts.
Colorbond will generally last longer than timber fences with timber posts, it has other disadvantages. The heat from our hot Australian sun is absorbed into the steel and radiates onto plants close to the fence which can be harmful to the plants depending on how well the plant handles increased temperature.
Also colorbond fencing is easily bent or dented by kids, animals or vandals. For this reason I do not recommend a colorbond fence that backs onto a public area because vandals seem to love tagging them, (spraying them with spray paint) or kicking and denting them.
Although there are over 32 colours to choose from with your colorbond fencing, neighbors often do not agree and it can be costly to paint one side a different colour.
In summary both colorbond and timber fences have their advantages and disadvantages and your personal circumstances will depend which fence type would best suit your needs.