I have a rubber plant with four branches growing out of the pot. It has grown about three feet in six months. I am very happy with it but am concerned that the branches are very thin and need supporting. Is this normal.? Also the dominant branch is rapidly approaching the ceiling. How and where can I prune.?
Yes the rapid growth can be normal with good soil fertility and watering. But growth may be excessive if you over-fertilize.
Without photos its hard to advise on exactly where to make a height reduction pruning cut, but in general you should look for a lateral branch off the parent stem, and cut just above the point of juncture. If no laterals, you can cut mid-stem, but it will be an ugly stub for awhile until new growth appears. The other option is to remove the entire tallest stem and allow the shorter ones to remain.
Or stake the stems for artificial support.