You don't want to blister and char your guests' fingers! Set the temp to the lowest possible - usually 170F - 200F. That will be too hot to handle but will cool a little by the time you plate and serve.
How long it takes a plate to heat up depends a lot on the plate. Corelle is light and thin while stoneware is thick and heavy.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan
My toaster oven hold a full size plate. If I'm heating a plate I either stick it in there on as low as it goes, 170F I think, or on top of the stove if there is room and the oven is on. 170 is still too hot to handle IMO, so I don't leave it in long enough to reach 170.
Like Andy said, a lot is going to depend on the plate.
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I have put them in a sink of really hot water pull them out dry and set on table. Kind of a pain but it dos work. I just had a thought what if you took hot dried towels from the dryer and stacked plates between them then stick back in warm dryer. Don't turn it on of course
"It's so beautifully arranged on the plate - you know someone's fingers have been all over it." - Julia Child