Diacetyl rest?

Hello at what point during the fermentation does one want to do a Diacetyl rest.?

I can only speak for my preferred fermentation profiles.  I really only do a diacetyl rest for my lagers.  I ferment at around 50F for 7 to 9 days (dependent upon yeast strain) and then ramp up slowly from 50F to around 60F over the course of a week.  I then bring the temperature up to 64F-66F for several days (4 to 5) before slowly bringing the temperature back down over the next 4 days and cold crash for 4 to 5 days.  Following that, I will bring the temperature up for bottling, but at that point the lighter styles are usually pretty bright and I've never had any hint or feedback of diacetyl.

For ales, I start my fermentation in the mid to low 60's and allow it to rise slowly after the first couple of days up to 68F to 70F.

There are a good number of ways to go about fermentation.  If you google 'lager fermentation' you should come up with more different ways than you can imagine. (not to say they all will work for you).

I generally start the ramp up to diacetyl rest once I reach 50% attenuation. But, I only do that if necessary. I'll take a small sample and smell/taste for diacetyl.

Sometimes I use the below as a general guideline. This isn't my work but found it on the Net.

OG of Wort Yeast Type Approximate Primary Time
≤ 1.060 OG Liquid           4-7 days
≤ 1.060 OG Dry                   5-8 days
≥ 1.061      Liquid           6-10 days
≥ 1.061         Dry                   7-14 days