That... sounds very unlikely. It didn't take long for me to get a proc at all.
No I can't make logs at the moment, the reason I have so many issues providing proper proof is because I need to take a recount screenshot. Unfortunately, you can't see if the Multi-Strike procced from the auto-attack or Serpent Sting tick I did.
EDIT:
Wait I just got something, Give me a minute to upload it.
EDIT2:
I put Serpent Sting on 4 dummies, then cleared my recount. After all data was cleared I had multiple procs from Serpent Sting dots. This was as Beast Mastery as well.