To activate properly the Subsurface scattering in Substance Painter a few parameters have to be set first.
This page provide a guide on which parameters to enable.
1 - Texture Set Settings
In the Texture Set Settings add a Scattering channel if not already present :
The Scattering channel works like a mask over the surbsurface : if the channel is black there is no subsurface at all, while if it is white the subsurface intensity will be at maximum. This channel is a grayscale value that is black by default. Add a fill layer in the layer stack to control the default color or use a paint layer to manually control the intensity.
2 - Global Subsurface Setting
Enable the main Subsurface scattering setting in the Display Settings (below the Post-Effects settings) :
Enabling/disabling the Subsurface effect affects the whole project. It can be helpful to use this global parameter if it is too heavy in terms of performance.
3 - Shader Settings
In the Shader Settings window with default shaders can be found a "SSS Parameters" group with two settings.
Change the scale and the color to fit the target material. For more details on these settings see : Subsurface Parameters
Bonus : Enabling shadows
The Subsurface scattering effect works well but may look strange if alone.
Enabling shadow can help the final look in the viewport and improve the realism of the final material.
In the Display Settings window, enable the "Shadows" setting :