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To tell you the truth, there is no way to detail it that well. Some people can be looking for minutes while others can be looking for months and never run into one.
I mean, sure, you can get averages, but unless you get everyone in the world who plays pokemons average time and where they found them your best bet is to just look like everyone else.
Getting details from other people will not help you all that much as it varies.
There is no way to find one. They appear everywhere, no pokemon made them appear more frequently, and you cn spend seconds or years until one appears. Sorry :(
Do the soft-resseting trick on guys you have to talk to to obtain/battle. Same time to get one, but easier process. I got Regigigas in a day and Rotom in a month (I tried every other day for about 30 minutes.)