To get the huge number. It usually takes about 15-20 minutes to get another one if you remain relatively focused on the task.
Original goal had a minimum of 18 without having any for reset. This number may make some OCD sense:
-One to remain in the collection of legendaries in the main (original) Diamond game.
-One for the living pokedex in the MPR Pokemon Pearl cartridge.
-16, each holding a different plate for the living pokedex expansion in the MPR Pokemon Diamond cartridge.
Going above the count of 18 is only natural from there. Having at least two to soft reset is great... a gimmicky one to take advantage of its unique moves and at least one with a more serious design.
It's plenty easy to bump the desired count higher still if you want to hold on to extra ones you can trade for people.
Now, as I said, my ending total will likely be 25. On Saturday, I had a total of six games that could receive Arceus:
-Main Pokemon Pearl
-Main Pokemon Platinum
-Resettable Pokemon Diamond
-Resettable Pokemon Pearl
-Resettable Pokemon Platinum
-MPR Pokemon Platinum
Of the resettable games, Platinum is not in a state that can be reset thanks to the Membership Card and Oak's Letter. That leaves the resettable Diamond and Pearl games for obtaining more Arceus. For each subsequent visit to Toys R Us, I can prepare those two games ahead of time and thus we can consider those Arceus to be freebies. Those visits would be:
-Sun, Nov 8th (2nd day of the event, to reach the 18 minimum.)
-Wed, Nov 11th (Not as much pressure to get home immediately.)
-Sun, Nov 15th (When picking up New Super Mario Bros Wii.)
That's 2 * 3 = 6 free Arceus, plus the 6 from the first day meaning 12 "free." Given the current target of 25, that leaves 13 to get. 13 * 15 = 3.25, 13 * 20 = 4.3 so about 4 hours if undisturbed.
Now, practically speaking, it's a rather boring 4 hours so it's incredibly easy to be distracted, thus it stretches into the 4-5 hour range.
It's only natural.