I don't think they're ever deleted unless the author decides to redo one for some reason. Good question as to how many; I know for sure there are hundreds and would not be surprised to find there are over a thousand. Oh wait, maybe you were asking what is the average length of the videos? IM Danny Rensch recently asked the presenters to limit them to 15 minutes, but there are lots of much longer ones. My best guesstimate is that the average one is 20 to 25 minutes. A lot of the better ones are multipart videos though, especially the ones on openings.
I find them useful especially as an opening reference, but I can't keep up with them all. If you do go diamond, check out IM Danny Rensch's Live Sessions. Great edutainment!
how long do the teaching videos usally last in time, and how many are avalible if I join.