Where to even begin, haha. I collect Pikachu, Seviper, and the Moltres WotC #21 artwork in all languages and variations. The cards I’m still missing for those collections I’ve been searching for since I started collecting those, so 5+ years for Pikachus, 2.5+ years for Seviper and 2+ years for the WotC #21 artwork. When I joined the forum at the start of 2016 I had around 250 of the at that time 650+ known Pikachu cards; at the end of 2017 I had around 700 of the 825+ known Pikachu cards; and right now I own roughly 1200 unique Pikachu TCG cards, and missing around 55-60 to ‘complete’ it. For Seviper I still need just two cards: Japanese unlimited edition World Championship Pack and Spanish Hidden Fates - Shiny Vault. And for my Moltres WotC #21 collection I’m still looking for the Australian sealed version with Game Boy Color insert, as well as both a 1st and unlimited edition Japanese Pokémon Web booster pack.
I used to look on multiply different eBays (.com; .co.uk; com.au; .ca; .de; .fr; .it; .es) a few times each week, and on CardMarket once a week. These days it’s a bit less, like once every two weeks on eBay and once a month on CardMarket.
To give some examples of cards I recently acquired after searching for them for a long time: Italian Reverse Holo Mysterious Treasures Pikachu; Japanese unlimited edition World Champion Pack Battle Frontier (this one is currently at my middleman); Inverted WB stamp Pikachu; etc.
And to give two opposites that didn’t took long to find: when I decided to add all Jumbo Pikachu cards to my collection it only took three months to find all -at the time- sixteen of them (including waiting for them to arrive). It was the perfect moment to start collecting them apparently. 
Also, when I decided to collect the entire National Index including all forms and Shiny Pokémon, it only took three months (mostly waiting for cards to arrive) to fully complete it. And yeah, in those three months I’ve spend quite a bit of money considering the Shiny Pokémon of Triple-Star and Gold Star Secret Rares…
Greetz,
Quuador