Bertho Boman experimented with various concentrations and presented results on his blog:
This time I tested several different plastic types and varying strength Sodium Hydroxide.
1.0 Molar: 40g per liter water
1.8 Molar: 72g per liter water (same as the video)
3 Molar: 120g per liter water
6 Molar: 240g per liter water
After 5 hours in the ultrasonic tank @ 60°C there was no visual effect or surface hardness change evaluating the samples under a microscope of the different plastic types except the PLA.
Extensive details and experiments with other methods and how to dissolve PLA fast when no other material is involved:
Here is video showing the process with professional ultrasonic tank and caustic soda (sodium hydroxide) that takes 3 hours to remove PLA support material from ABS part. 217 grams is added to 3 liters of water heated at 60C.
Ultrasonic tank used in the video is priced around 1300 USD, but it can be found much cheaper at some 300 USD.
They talk about dissolving PLA in .stl podcast (great podcast btw):
RepRap forums discussion: