This fence/shed paint has a powerful pigment and waterproofing element. It produces a sheen when you use it - two coats please! :) Do it right and 3 years without even looking at it is normal, even here in Cumbria, where it rains 13 months of the year... The tins say 6 years - I tend to be more cautious but somewhere a little more sheltered than on the side of a mountain and I can see that would be within reason. It does need 2 coats - you also need to make sure the second coat goes on before the first entirely 'sets' else you come a cropper against the waterproofing. The final coat effectively seals the wood and you can see the water beading on it.
It is unbelievably expensive - but when you consider the cost is spread over 3 - 5 years, it's not so eye-watering, and it does look blinking brilliant.