EDIT: Excuse the wall of text incoming below… 
Tbh, the moment I started collecting as an adult was immediately serious. 
I used to collect Yu-Gi-Oh cards and bought cards from TrollAndToad and some other websites pretty often. On a whim I decided to buy every Pikachu card below 25 USD they had in stock, which was basically the start of my serious Pokémon collecting career. That purchase on a whim wasn’t serious, but the months that followed were. At that time I started looking at all the Pikachu cards that existed, and also decided to expand into foreign languages.
I could never have imaged I would hold the largest Pikachu TCG collection in the world for a period in time. When I joined this forum near the end of 2015 I had around 250 Pikachu cards in my collection, whereas justinator, who has been a huge inspiration to me and who was the reason I joined this forum, had already over 550 of the total 650 he knew to exist. At the end of 2016/start of 2017 I surpassed him when I reached 700 Pikachu TCG cards (and he stopped shortly after unfortunately, because he bought his first apartment and couldn’t afford it anymore). Near the end of 2018 I got surpassed myself, since a fellow Pikachu collector and online friend of mine from Belgium (Tom) has about 35-50 more Pikachu cards than me. I now have little over 1250 Pikachu cards, and he around 1300 (I think).
Two other things already mentioned are also very true:
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When I was collecting Yu-Gi-Oh cards the most expensive card I ever bought was 25 USD (Secret Rare Black Rose Dragon). Since I usually bought cards for less than 5 USD, that 25 USD was already pretty expensive for me. I therefore also thought about it for at least a week before deciding to buy it.
Nowadays I would buy any Pikachu I don’t have yet (unless released yesterday) for 25 USD without a second thought, and I’ve even bought three cards thus far for over 3k USD individually, as well as 5+ over 1k USD.
(Keep in mind that I at the time of the 25 USD purchase only had a simple job as Saturday postman, whereas I now work a 40-hour job as Software Developer without much other expenses.) -
What Scott said is also very true! Although less now, I used to look on eBay and CardMarket at least five times each week looking at new Pikachu cards that had popped up (which also paid of a lot tbh). These days, since I’m reaching the very hard to find last cards to complete my collection, I don’t do it as much anymore. I do still look on this forum at posts on a daily basis though, as well as the /r/pkmntcgcollections sub-reddit.
I think the amount of time you spend says a lot more about how serious you are than the amount of money spend. But then again, they say time = money, so I guess the combined amount spend really says it all. And tbh, I don’t even want to know what I’ve spend in total in terms of both time and money thus far, haha.
One more concerning aspect for me personally however: I think I’ve spend this past year 2019 about as much on all my collectibles combined as I’ve earned with my job that entire year (minus daily expenses). Basically, my savings account remained on the same level from Dec. 2018 to Dec. 2019 after working 40 hours a week all year around. This really made me second-think some things this year, and how this will continue in the future. 2019 just had WAY TOO MANY Pikachu releases. At least 250 new Pikachu TCG cards were released across all 14 languages in 2019 alone. This is absolutely crazy, considering there are around 900-950 Pikachu releases in the prior 21 years combined (43-45 a year on average)… 
I’m thinking it’s a waste to stop now that I’ve come this far, but at the same time I’m considering to pause some of my collections including Pikachus, and only focus on the ones I right now get the most joy out of: Yu-Gi-Oh rarity collection and Pokémon Full Art Supporter collection. For those who don’t know, here a summary of all the things I collect right now at the same time: Pikachu (all languages & variations); Seviper (all languages & variations); Moltres WotC #21 promo artwork (all languages & variations, including sealed, PSA-10, and BGS-10); Full Art Supporters (English & Japanese exclusives); Full National Index including all forms & Shiny Pokémon; Yu-Gi-Oh Rarities (TCG; OCG; Korean; and misprints); twisty puzzles. And I used to collect Mimikyu; euro coins; and minerals & fossils in the past as well, but stopped with those. I’ve now also stopped with twisty puzzles and started selling some, and been seriously doubting about pausing my Pikachu and National Index collections until I’ve saved up a bit of money again on my bank account. In the past I used to put aside about 900 USD when I received my wages, and never touched my savings account for my collectibles. After buying a few 1k+ USD cards where I had to resort to my savings account, I did it more often every time my personal bank account reached below 100 USD again from PayPal purchases. So I will now go back to my previous strategy of saving money over time, and will not buy as much as I did this past year. I know most of you already save up money and carefully think what to buy (duhh…), but I personally earned enough money with my job in the past to not have to worry about that. Until now. Ah well, we’ll see how this will continue I guess, but for now I’ve paused my Pikachu and National Index collections. I will of course still buy some cheap Pikachus from my wanted list when they pop up, since I’ve been looking for them without luck for the past 4 years (so it would be a waste to ignore them if they do finally pop up). But I won’t buy any new releases for now.
I guess I’m currently a bit of a too serious collector for my own good.
Greetz,
Quuador