Just recently finished mine.
If you just want 1 of everything then it is pretty easy, just catch loads of stuff on your run-through of a few games and breed the more desirable ones for simple GTS trades. If you have a rare vivillon pattern then this is easymode.
Then you just need the mythicals, either wait for events or browse the web for trading sites (or even just ask around on wifi general
>>>/vp/wfg or, if you dont mind hax, injection general
>>>/vp/injection ).
If you want all the Vivillon patterns then you must trade for them, you can catch all the various Unown forms in OR/AS or G/S/C on the Virtual Console (minus the punctuation ones).
Depending on your starting point and dedication this could be done in under a week or a few weeks.
My livingdex took years but I often took breaks for months at a time and autistcally wanted good IV/Nature/Ball combos and one of my own OTs on every pokemon.
