There are several places in Dune where the word "Thumper" is used as a measure of distance:
On the north polar map of Dune, there's an arrow pointing to the left with the caption:
20 Thumpers to Palmaries of the south
After Paul summons and rides his first sandworm:
Paul studied the cliff, the great streaks of rock crossing it like waves. No green, no blossom softened that rigid horizon. Beyond it stretched the way to the southern desert – a course of at least ten days and nights, as fast as they could goad the makers.
Appendix I The ecology of Dune
Kynes went down to the palmaries himself – a twenty thumper trip (in a palanquin like a wounded man or Reverend Mother because he never became a sandrider).
The arrow on the map starts at about 60°N and from the cartographic notes for the map, we know that the palmaries are at 40°S, so 20 thumpers subtends about 100° of the planet's circumference.
From the "They have tried to take the life of my son" chapter, we know that Dune's surface gravity is 0.9g:
'Hey! Feel that under your dogs? That's gravity, man!' 'How many G's does this place pull? Feels heavy.' 'Nine-tenths of a G by the book.'
From that, I'm assuming that Dune is roughly the same size and constitution as Venus which also has a surface gravity of about .9g, so an equatorial diameter of 12,092 km and circumference of 37,988 km. So a thumper would be approximately 527.6 km (or almost 328 miles).
In the Terminology of the Imperium, a thumper is described as:
THUMPER: short stake with spring-driven clapper at one end. The purpose: to be driven into the sand and set "thumping" to summon shai-hulud. (See Maker hooks.)
An interpretation might have been that a "thumper" is the distance at which it can be heard or sensed (by sandworms). However, it doesn't seem likely that a small mechanical device could be heard at a distance of over 500 km.
What does the word "thumper" mean when used to measure distance?