What’s the longest you’ve spent on getting one design to work?

[Wizard Kerbal]

I realized I started working on one of my designs the winter of 2020 (I still don’t have it flying), and so I wanted to ask other kerbonauts: what is the longest you’ve spent getting one of your designs to work, let alone perfect? 
 

About that design, it’s a cargo VTOL that uses 6 electric engines to lift it and <50 tons of cargo into the air across the planet. It has so many little parts that have to work right that are easy to not work right, and it’s just annoying!

(6 engines, 48 propellors, 3 rotating clockwise with 24 propellors with a certain pitch, 3 counterclockwise with 24 propellors pitching a certain way, 2 of those engines on rotating servos that have to be mirrored but the engines can’t be on symmetry though, and and and . . . )

Edited July 12, 2022 by Wizard Kerbal

[Rutabaga22]

I spent a whole day refining my first mun mission

[king of nowhere]

It usually takes around a month to perfect the hardware to run my large missions, possibly more. They are not a single system, though, but a collection of a mothership to carry everything around and a bunch of specialized smaller ships to perform specialized roles.

For a single system... it probably took a couple weeks to perfect my Dancing Porcupine's armor and robotic joints. Nowdays I use much simpler models, though.

[MSteele]

Don't laugh, but I'll spend days working on a Sounding Rocket.

[SkyFall2489]

I spent several weeks trying to figure out why a high performance SSTO was spinning out of control during ascent before I found the issue to  be engine flameouts. Turns out, the devs made the RAPIER use more intake air but never told anyone. If it counts, the project has also been shelved for at least a year...

[Dinlink]

I have spend several days and +1MM funds in hard mode with Ferram Aerospace mod trying to make a 30 parts nuclear rocket propelled airplane (+5000 DV) without success... I can't take off because stability issues xD... I will keep trying until game bankruptcy xD...

That's the most time and funds I have spend in a project... And is still ongoing

[magnemoe]

Spent some days on some bases or Eve accent rockets. 
Now stuff like the standard base I tend to modify over time and depended on needs like out at Jool solar don't cut it. 

However the worst was then I came in fast at Duna, I realized I would come inn much faster than I expected so I had to do lots testing and move stuff around in transit to get the base and tug to be stable at 7 g aerobraking 
Firebird one at hardbrake. 

Pulling it off was awesome.