How many early Christians returned to plain Judaism?

The Book of Acts makes the claim that, in the earliest days of the church, "thousands" became followers of The Way (e.g., Acts 4), including "a large number of priests" (Acts 6.7, in today's BCP lectionary reading).  I gather the author was referring mainly to Jews. I'm curious about how many of these folks stayed with the church. At least some of them probably drifted back to a more-conventional variety of Judaism, or to nothing at all. I wonder what the percentages were in each category.