What is supposed to be the length of duct liner to be installed inside duct from machine for both supply and return side?

actually length of duct liner depend on the sound analysis, and how much sound damping we need so for analysis you will find the next variants:

- equipment size and selection, and sound data from manufacture.

- sound pass analysis.

- size of ducts and insulation.

- type of space and interiors.

anyway it is complicated process, as a maximum and minimum you can consider 3 m length minimum and 10 m length maximum these values obtained from previous projects data.

Both in supply and return of AHU or FAHU, the  Acoustic treatment/insulation shall be min3 meters internal duct lining1" thickKg/cubic meter density acoustic lining required from the mouth of the unit till the max meters. However in the machines like FCUs or package units where the Acoustic lining are to be provided up to1.2 meters or till1st outlet/inlet.

Both in supply and return.

For AHUs shall be 3 meters internal duct lining 1" thick 48Kg/cubic meter density acoustic lining required from the mouth of the unit. Both in supply and return.

You can consider 3 m length minimum and 10 m length maximum for any unit to reduce the noise level in the duct

thumb rule 2 mt rs for FCU and 3 to 5 meter for AHU,

for sounds which are beyond the audible limit restricted to the NC level in side the specific space you need to calculate the length required,Many cases you would require to use sound attenuator to damp the sound.

In order to calculate this you would require the duct size the machine technical detail for sound and the last point or diffuser and the distance of listener from it .

3m for supply and for return i think no need

First of all, you have to make sound analysis for your duct work. Then, you decide to go for duct lining or sound attenuater to eliminate machine sound. In case sound attenuater unit being costly, you can use up to 3 meters of lining for both supply and return ducts. 

Simple thumb rule to be followed, for FCUs 1.5 meters internal duct lining 1/2" thick 32 Kg/cubic meter density acoustic lining required from the mouth of the unit. Both in supply and return.

For AHUs shall be 3 meters internal duct lining 1" thick 48Kg/cubic meter density acoustic lining required from the mouth of the unit. Both in supply and return.

In Supply and return air duct  sound liner length must be 15 to 20 Rft 

ALL MAIN SUPPLY AND RETURN AIR DUCTS SHALL BE INTERNALLY LINED WITH ACOUSTICAL INNER LINER  FROM THE A/C   UNIT TO THE FIRST AIR OUTLET OR INLET.

It would be 15 to 20 ft depending upon the situation of the site.