That depends on the road. Hill country with lots of bends, rises and falls will require a close spacing, say 10m. Flat open country with very few changes in grades and direction may be stretched up to 50m or more. Urban roads with kerb and channel may be 10m, 15m or 20m again depending on the grades, curves, features, etc.
So you need to assess the stretch of road to be surveyed and decide what will give a reasonable result.
regards
sc