I know of no Rock Art site which has been rejected. It is therefore rather difficult to identify the cut off point at which OUV ceases to exist!! There is certainly the impression that "Rock Art" is almost, by definition, regarded as being of OUV.
I note that "Rock Art" is in fact covered by 2 relevant Connections - one for painting and one for incision etc. How strict we have been with the assignment across and whether it holds up (e.g are all the sites one or the other?), I don't know. Also what is the significane of the difference- I am sure the periods at which mankind has produced each of them overlap - though perhaps the incision etc ones on the whole are eaerlier
a. Pictographs (i.e Paintings) with 29 sites - http://www.worldheritagesite.org/tags/tag410.html
b. Rock Art (i.e Petroglyphs involving removal of rock) with 39 sites
You have titled your question as applying ONLY to the latter - but is that a useful distinction?
It could be interesting to try to put together a grid of all the sites identifying Period/Age and Culture/Geog Location together with whether they are paintings or cuttings.. Those already inscribed do cover the entire world and the entire sweep of human development from earliest "man" to (almost?) contemporary art (in Africa) so, it is perhaps not surprising that there are so many?
ICOMOS does seem to have been particularly interested in Rock Art over the years and has sponsored various reports on the subject - it could also be interesting to revisit these and identify gaps in period/geography/culture which have been identified