There are lot of fieldstones in my area that it makes sense to fill the bench with them
and mortar them with clay and sand.
How many inches of this mortar should i apply around the ducts?I assume the fieldstone
is better to not touch and be in direct contact with the ducting?Am i right?

So  just test my soil with a soil jar test,see the ratios of clay and sand and  try to get
a good ratio for the mortar? What is the indication that the mortar is suitable?

Hi Panagiotis;  I put a 4" layer of straw filled cob down first , then I laid my duct pipe on that and covered them with maybe 2 " of regular cob , then I started laying my slate (similar to fieldstone)  A layer of cob then a layer of stone...  then a layer of cob again , I called it cob lasagna .I did carefully place my lowest stones to not put to much pressure on the ducting. Ultimately your ducting will rot out and you will be left with a smooth clay tunnel for your gases to travel.  When I built mine I believe I used too thick a layer of cob in between the layers of stone ... less cob is better ... just enough to cover the lower stone and enough to seat the new stone. I didn't understand that stone is a much better heat holder than cob... So less cob more fieldstone !  This is less work !

Thank you Thomas! This is how i want to build it.
Brick casing with field stone as the thermal core and mortar to fill the space between the stones.Wouldn't it better to put even more stone than doing lasagna? Just stone and clay sand mortar?
Basically this time of the year i go to the sea,the weather is still nice here ( 22-25 celsius) and i love how the stones between the sea and the land , reserve and slowly radiate out the heat from the sun. So much stone here to acquire lot of thermo mass quickly,why would i go for so much cob mixing?
Why do you believe that the ducting will rot out?Also why did you put the straw layer beneath?Do you want to insulare the space beyond and not wanting it to act as a thermomass?Do you want heat faster from the surface?

I put the straw in the base layer to insulate my pipes (heat) from the ground, otherwise I would be trying to heat the earth underneath rather than that heat coming up and heating our studio.The earth is to big to use as thermal mass... Ha Ha...  Other than the very first pipe leaving my transition area , my ducting is very thin hvac pipe , very cheap and comes in 5' lengths. No need for expensive heavy wall pipe after the temperatures come down. I'm thinking 10-15 years before my thin wall deteriorates but eventually it will. The cement board you see at the back of my mass has 4" of perlite behind it to insulate my heat from the stone foundation. If I allowed my mass to heat that stone it would steal  my heat and transfer it outdoors... I want all that mass inside insulated from the floor and outside wall, keeping my heat inside the room!  Yes , less cob more fieldstone is better but its still lasagna.

Panagiotis; When you are encasing your mass with brick on all sides then the consistency of your cob is not as important as it is when building an exposed cob bench. Almost any clay/mud would work. Here's a few more pics  to encourage you .