It takes about an hour tops if you have the Power Items, even with no vitamins and no Pokerus. With vitamins, it usually only takes me like half an hour.
Say you need 252 in a certain STAT. If you can find something that gives +2 and has a high appearance rate, each kill with the right power item will give you +6. That's about 40 wins, which often just means going through all your PP, going to a Pokemon Center once, and then coming back.
But uuuuusually, I'll throw a lot of vitamins in first. Then that same Pokemon only needs about twenty wins to complete training in that STAT, which takes no time at all.
After that, the new experience system makes it a breeze to catch the Pokemon up to level 50 or 60 with Exp. Share and switching out. It's only for the last thirty or so levels that things get tedious.
Come to think of it, why haven't they let us create custom Pokemon out of hammerspace yet? I mean, they have it on NetBattle and PO.
Because the online simulators don't capture a major aspect of the games: discrepancies in effort. The actual Pokemon metagame is about strategy, luck and effort (training, breeding and raising). The simulators remove a full third of that equation to create a very different experience that the actual franchise will never support.
There are EV reducing berries for that.
Although I will say these are currently way too hard to come by to be useful.