‘And after they had spent some time there, they were dismissed in peace from the brethren to those who had sent them forth.'
Then once they had spent some good time there, they were sent back to their own church with expressions of peace and goodwill from the Christians of Antioch which were to be borne to their brethren in their sister church.
In view of Acts 15:40 it may be that ‘they' here means a Jerusalem party who had come along with the two, and that Silas remained behind. But there is no hint of that and there is really no reason why Silas should not have returned with Judas, in order to report back to Jerusalem and then later have returned to Antioch, either wholly of his own volition, or even because called on specifically by Paul when he recognised that he would need a new fellow-worker. The time scale certainly allows for it.