Where does that assumption come from? No Russian source has ever said that. The only sources I’ve seen were the US propaganda arm. Putin said that they were now in a defensive war with the west (his words, not mine). I don’t think they expected that to be over any time soon.
But just for fun let’s assume you’re correct. Their objective is to conquer Ukraine..?
It still doesn’t change the fact Ukraine isn’t winning. They’ve lost territory, most of the military assets, and the territory they’ve lost thus far is their most productive (industry and agriculture is based around the coast). By no measure is that trending toward victory.
Now my position on this — Recall, Ukraine had one of the largest standing armies prior to this war. Order of magnitude more prepared than Iraq in the 90s or 2000s. With better equipment, heavily entrenched and much larger by land mass.
I don’t think there’s any way that Russia expected to conquer Ukraine with 50k troops (1/5 the size of Ukraine’s standing army) in 90 days. But I also don’t think Ukraine is the only theater of battle. The real war is the war of logistics and in that, Russia is far better prepared and capable.