وَالْقَعْدَةُ الْأَخِيرَةُ مِقْدَارَ التَّشَهُّدِ
Meaning: and sitting at the end for the duration of the tashahhud.
The final sitting is integral, and its minimum is measured by the time required to recite the tashahhud, not merely touching the ground in a seated posture.