Question:
Answer:
Each local parish seems to have its own custom in this regard. The General Instruction of the Roman Missal states:
. . . as circumstances allow, they may sit or kneel while the period of sacred silence after Communion is observed (43).
In 2003, the Congregation for Divine Worship and the Discipline of the Sacraments said the following about GIRM #43:
The [intention] is that that the prescription of the Institutio Generalis Missalis Romani, no. 43, is intended, on one hand, to ensure within broad limits a certain uniformity of posture within the congregation for the various parts of the celebration of the Holy Mass, and on the other, to not regulate posture rigidly in such a way that those who wish to kneel or sit would no longer be free (Prot. N. 855/03/L).
The Vatican is saying that, while uniformity of posture is a good thing, individuals are still free to sit or kneel after Communion depending upon their personal preference. You are free to sit or kneel the moment you return to your pew or at any time until the congregation stands for the prayer after Communion.