I have never seen a schedule for upcoming CUM PTFs.
If I have had a serious problem in the past and have been working with IBM on it, then they may give me an estimated date.
Here is a web site that will let you get to the latest information.
I found that if you order SF98vrm (in my case SF98540) that the cover letter states the anticipated next release of CUM PTF’s. Not eactly what I was looking for, but it works.