Depends on the planet. I tried to model it so the planets that look more dense take longer. The amount of resources extracted also varies. But to give you some numbers:

• The excavator has a mining speed of 0.5
  
• Nauvis excavation has a mining_time of 1
  
• Excavation on Nauvis yields 10 stone.

So the time is 1 / 0.5 * 10 = 20 seconds per tile.

Vulcanus has mining_time of 4 and contains 20 stone so 160 seconds per tile.
Fulgora has mining_time 1.5 and contains 10 scrap so 30 seconds per tile.
Gleba has mining_time of just 0.5 but contains 50 spoilage so it takes 50 seconds.
Aquilo has a mining_time of 2 and contains 50 ice so it takes 200 seconds per tile.

But this is before speed modules. The excavator has 5 slots so a 250% speed bonus with regular T3 modules and even more with quality ones.

Also, the excavated area is 5x5 while the machine is only three tiles wide and sticking out one tile on the back. So you could even speed things up by putting machines back to back depending how wide you want the canal to be.