Try PIHKAL or Future Drugs.
http://www.4shared.com/folder/nM4gT-vL/General_Synthesis.html
Theres also a new book he has released I havent found a PDF of this yet.
I have a copy of his lab notes ill put them up there also.
You could also try the "Ask Shulgin Online Stuff"
I have heard this quoted also. I actually think three months should be the absolute minimum time between roles.
Casualties are reported with more frequent use.
The other option is you could try and email shulgin himself.