Road traffic signs do not all apply over the same distances. A road traffic sign is governed by the condition of the roads or area, for example no overtaking may only apply to an area where it is not safe to overtake, or a reduced speed limit sign may only apply in a high accident zone. Road conditions vary in many ways and at different points in a road, and it would therefore be impossible to have a sign apply over the same distance at all times.
Road signs generally indicate for what distance the law is applicable. If it does not, the road sign will remain in force until the conditions change and a new road sign governs the new conditions.