This depends on the chosen product. The less film and pigment on the surface, the shorter the durability. As a rule, opaque products have the longest durability. The wood construction also plays a vital part for the durability, and rain water must be able to escape rather than settle on a surface.
Many factors play a part in the durability of wood protection: Weather, application, quality of the wood. Wood is an organic material with many variations in both structure and appearance. In general, the majority of the problems are related to the construction.
If there are too long intervals between treatments, and the wood protection has started to peel off, your wood is completely open and unprotected. This means that the wood is much more exposed to wind and elements and it will degrade much more quickly. Moisture will penetrate into cracks, creating perfect conditions for rot and mould. If there is moisture in the cracks, and the temperature is below 0°C, the wood fibres will be destroyed, and wood will suffer even further decay.