Diacetyl Rest Questions

Sorry for posting these questions in a second forum, but I haven't gotten any nibbles over in the Beginners' forum. I have a few questions about diacetyl rest:

How soon after the end of fermentation does diacetyl rest have to take place? If it's a few days later as opposed to right at the end, is that okay?

Also...are there downsides to diacetyl rests longer than 24 hours? Does something else start going on that can harm your beer if the rest is 3 or 5 days instead of 1? And...is 1 day of ambient temperature at the recommended level enough to even get the beer to that temperature? Seems to me if I have 5 or 6 gallons of beer and take the ambient temperature from (say) 48 to (say) 58 for a day, it's unlikely the beer temperature has gotten up to 58 for very long.

Thanks for any help--I'm loving lagering!

Thanks--some pretty helpful info in that thread. I checked the SG of my two lagers two nights ago and both were about 1.022. They are fermenting at 48 degrees ambient temp in a freezer. So it seems like now is a perfect d-rest time; according to a quote from Noonan in that thread "An entirely different approach is relatively common in modern ferementation cycles. When the density drops to about 1.5 ºPlato (SG 1.006) above the target terminal gravity, the brewer raises the temperature of the post-kraeusen beer to 52 degrees F (11 degrees C) or higher, and holds that temperature for two to seven days for a diacetyl rest to reinvigorate the yeast culture so that it will metabolize diacetyl, removing it from solution."

And the added benefit from a successful d-rest using this method is apparently a reduced lagering period. So when I get home today I'll raise the temp (hopefully I am still in the 1.018 range or so) (I'm using Wyeast 2124 that suggests 58 degrees for 24 hours; the other one is Wyeast 2308 which doesn't specify but says it will benefit from a d-rest); hold it until maybe Sunday; rack the beer; and start the lagering process.

Well I seem to be "stuck" at about 1.020. I looked at https://www.homebrewtalk.com/f37/1-020-curse-63896/ thread and figured I would give both brews a gentle stir and a temperature increase. I wonder if it's wise to just do the stir for now and see where things go. On the other hand it's about time for the d-rest anyway which would require the temperature increase. Hmmmm....