There are two studies mentioned in FrankTalk that I found credible (both were published in peer-reviewed medical journals used by physicians all over the world).
As I recall,
Both recommended 60 days pre-op therapy as a minimum.
One rcommended 30 minutes once or twice a day pumping up to a full erection inside the cylinder and holding there (DO NOT OVER-PUMP!) and noting the size you get for record keeping purposes.
The other recommened 30 minutes once or twice a day pumping up to a full erection inside the cylinder, holding for less than minute, releasing the vacuum to return to a flacccid state and then repeating for the 30 minutes.
The former is easier to do. The latter has the advantage of flushing blood through your erectile tissues. Both tend to restore or maintain elasticity of your penile tissues and size.
Recently lost size MAY be recoverable, but long-time lost size, not so much.
Links to both studies have been supplied several times in FrankTalk, but I don't have them at the moment. Sorry. Do a search and you will find them, I am sure.
EDIT: I re-found this. I hope these are informative and helpful.
http://file.scirp.org/pdf/ASM_2013012514045982.pdfhttps://www.ncbi.nlm.nih.gov/pubmed/24506138See also this thread
viewtopic.php?f=4&t=11516