Hi Malcolm
as I understand it the status of a nom. prov. lies outside the ICBN rules http://www.iapt-taxon.org/nomen/main.php and therefore there won't be a time limit on it.
Anyone who is a regular on ASCOFrance will be familiar with people finding what are probably new and undescribed species. A provisional name gives at least a handle to further work on a taxon. If the finder or expert in the group has given it such a name one can also call it Xxxxxx xxxx Author ined.
I suspect that the modern way will increasingly be that new species are not formally described without sequencing; that will presumably add to the inevitable backlog.
It actually doesn't need to be too much of a problem really - thank heavens for the 'nom. prov.' system.
I shudder to think how many nom. prov's Zotto has got(!) but I would be the last person to put a time limit on him . . . . .

best wishes
Chris

Thanks for the info Chris.
Mal