What’s the longest time you’ve waited before adding something to your Pokedex that’s been available for a while?

That would be my Blastoise, which I've gained as a Wartortle from a raid. I want to get anything that has at least 15 attack IV and is within the 80-100% range.

Others on the list that I could have but I'm waiting out on are:
Venusaur
Espeon
Dodrio
Primeape
Belossom
Sunflora

Unevolved forms of the above are available either now or from time to time around as nests (or raids in case of Ivysaur), but still haven't found the ones worthy of using the candy on them.

That depends a little on what you mean by "waited." I just finally evolved my first Flaaffy and Ampharos last night, which were the last two Pokedex entries I still needed for my region. I could have done it a long time ago if I'd spent like 90 rare candies and evolved one with ~70% IVs, but I waited until I had a better one and it wouldn't be so costly. It still cost me 15 rare candies, but I wanted to complete my Pokedex before more of gen 3 came out.

However, if you mean "waited" as in you had all the candies you needed and could have evolved one at any time, but just didn't do it, Vaporeon stands out for me as probably the most significant one, even if not longest in duration. Early on, I was saving as many of my first dex entries as I could for a double exp event, but I largely missed the first one in late 2016 since I only played like an hour a week before that. So then once the second double exp event finally happened (When was that, Valentine's day, Easter?) I had like 50 pokemon ready to evolve for the first time for the extra experience. Most of them were random junk, but not all, so yeah, I didn't evolve my first Vaporeon until I was around level 30, even though I had plenty of candies long before that.

There's been two Pokemon that I've had enough candies to get, but didn't and that was Omastar and Dusclops.

Omastar was the last one I needed to complete my Pokedex before Gen 2 arrived. I had the Omanyte candies but wanted one with high IV. In the end an Omastar appeared close to my house and I just had to go and get it.

I waited with Dusclops because I wanted to evolve a Duskull if/when we got a double XP event. Was going to wait with Shuppet as well, but a few Banette spawned at my apartment so I couldn't resist catching one.

I got my first Dragonite in March, right now have 2 at level 36; 1 at 35 and 1 at 32. Have 120 candies in case a high level+ good IV wild dratini appears...it won't, although I do see them quite often in dried creeks and ditches very close to where I live. 8 months to get 1; next 8 months 3 more plus somewhat powered them up with some extra candy ready

I still don't have a Hitmonchan.

That would be Espeon, who is still not registered in my Pokédex to this day. It's tough finding that perfect Eevee... :/

Pichu. Slippery lil' bugger.

It was some months ago. But for me it was muk. Had over a hundred candies but no wonder. Then raids came out and I just got one that way. Now it seems like every grimer I come across is a wonder. Go figure!

Those gen 3 ghost evolutions, waited for double XP.

I still do not have Pupitar. It doesn’t seem like I should waste 25 Larvitar candy just for a dex entry. I have caught or hatched about 8 Larvitar and caught about 6 Tyranitar in raids.

I have a 13A/15D/13S Larvitar and 166 candy. Could add two entries now, but would rather power him up to L30 right after evolving, so walking him another 120km. With the risk that Bite may become a legacy move for Tyranitar, I am seriously tempted to use 24 Rare Candy to get him now.

Lapras... Before the summer Fire & Ice event, I never found or hatched one. During that event, only 2 and the keeper was only 1140 CP and low IV.

What a fine D'nite, congrats! You will enjoy battling with him.

As for me, I waited the longest time, until late November, until I added my very first legendary, Suicune (so far, I've done exactly two legendary raids!) I'm very proud to have him, and hope to add Ho-Oh soon.

Better late than never ;)

I still don't have a Dragonite and I've been playing since the first week of the game nearly finished with level 35. Dratini are very rare in my biome and I have never hatched one >90%. I'm very picky about how I use my dust and I don't want to waste the candy just for the dex on a Dragonite that doesn't have good enough IVs to star dust. Someday.

Unown. Finally caught it in August during a Safari event in Vienna. My longest wait for a Gen 1 Mon was Lapras, which I got during Thanksgiving last year. Never had a really long wait.