Okay, gents, I just conducted a detailed review of every homebrew I've made since 2005 that had significant diacetyl. This resulted in 6 hits for me since 2005. The following yeasts all exhibited diacetyl for at least 3-4 weeks after packaging:
In 5 out of 6 cases, the diacetyl was gone after 3-4 weeks of conditioning. The 1 exception was when I added sorbate at bottling to prevent excessive carbonation in the bottles. It actually worked, but the diacetyl lingered for probably ~6 months, and it took that long to fully carbonate as well, but the finished beer after all that time is actually very good and I still have a couple bottles left. But anyway.....
Seems to me that sorbate might help lock in diacetyl for anyone who might want that, assuming that you started with diacetyl already in the beer and just wanted to hurt the yeast so they can't eat it. I'd recommend force-carbonation vs. natural carbonation from priming because it took forever for the primed bottles to carbonate.
And the most common fermentation schedule that caused diacetyl for me:
6-9 days at 50-53 F primary
3-6 days at 60-65 F "diacetyl rest" -- ha!!
2-3 weeks at 32-53 F lagering
<3 weeks in the package at ~60 F
That last line is important because like I say, diacetyl almost always disappears within 4 weeks.
Cheers and good luck.
P.S. For the record, for those who wish to AVOID diacetyl, these days I don't fiddle with temperature changes so much, and I ferment a bit warmer than I used to, and I think those are some of the reasons I don't get much diacetyl anymore. For example, the maibock I just brewed with S-189 yeast at 64 F for ~6 weeks in primary exhibited no diacetyl at any point throughout fermentation. I'll be doing that ferm schedule again in future, with other strains as well.