approximate time for indexing a conference proceeding
Yes it is possible for a significant amount of time to elapse between the conference and appearance of articles on IEEE Xplore. For example, here is a link to one of my papers which appeared in the proceedings of an IEEE conference that was held in September 2016, and it was added to IEEE Xplore in the end of February 2017.

Note that you can actually see both the Date of Conference and Date Added to IEEE Xplore on that page as well, right below the abstract. So you could also look up some articles from previous editions of the specific congress that you went to, and see how much time it took for that congress in previous years (but of course it doesn't have to be the same this year).