The "days to weeks" estimates were based on an assumption that Russia's army was in fact as strong as it appeared to be on paper. If that was the case, then Ukraine would have been able to hold out for a few weeks at most, before eventually either capitulating completely or suing for peace.
It has turned out that Russia's military has a *lot* of rot, meaning that their actual strength has been just about equal to Ukraine. When two more-or-less equal armies go to war it's a recipe for a stalemate - there will be tactical victories on either side, but the strategic picture will largely remain unchanged for months at a time. And that's exactly what we've been played out in Ukraine so far, despite 100+ days of war, Russia has basically made no real significant gains. So, until or unless something significant changes on either side we are going to see a long war of attrition. Which is likely to drag on for a number of years but is unlikely to last for decades.
Quite so.
From reading a range of different sources it seems to me that there are two different Russias emerging. The Russian state is not doing too badly, the drop in volumes of oil exports has been more than made up by the price increases (even though Russia is selling oil to, for example, India at a discount) which means that the government can pay for its troops and the ongoing costs.
However the effects of sanctions will increasingly affect the civilian and general industrial capacity. The Russian state may have appropriated all the Boeings and the Airbuses in Russia, but as the spares (brake pads, turbine blades, generators, windscreens, fuel pumps, hydraulic bits and bobs, etc.) run out the planes will be grounded or Aeroflot's accident record will revert to that seen in Soviet times. This effect will be seen elsewhere, apparently even Russian sources have stated that several million people have slipped into the Russian definition of poverty since February.
The Ministry of Defence's update on 22nd June (Twitter feed at
https://twitter.com/DefenceHQ/status/1539473242678779905) suggests that some 55% of the original forces of the self-declared Donetsk People’s Republic (DPR) were killed in action or wounded, since the start of the year. This highlights the extraordinary attrition Russian and pro-Russian forces are suffering in the Donbas. As both the breakaway areas have introduced conscription for all men up to the age of 65 without exceptions for health or suitability it is likely that the actual casualty rate will be higher.
This is possibly why the Russian casualties don't seem to be quite so high as one would expect - Putin is progressing his 'special military operation' using proxy forces.
So this is all a long winded way of explaining that Putin can continue the war for as long as he wants — the Russian military is not yet on a war footing as Putin gambled he could take Ukraine as easily as he took the Crimea. This supposes that no events occur (palace putsch, illness, revolution in the streets) which change the game but I reckon the chances of any of these things happening to be small.
Putin may continue to make small territorial gains at great cost in men and materiel but he has already lost the strategic war. One of his objectives was to stop other countries joining NATO, but Finland and Sweden will soon be joining. The jump in German's military spending will mean that it will, in a year or so, have more than a dozen flight-worthy Typhoons and a couple of dozen driveable Leopards and be on its way to maintaining the sort of military force which matches its economic position.
Ukraine is already on a war footing - its tax base has collapsed and the Government's outgoings have skyrocketed. It is being financially supported by its allies in the West. It can continue to fight for as long as it has its allies' backing. In this sense its closest allies are the Baltic states and Poland and these will do all they can to stop any of the others from backsliding. The UK is slightly different - it already has some significant grievances with Putin - Alexander Litvinenko was a former officer of the Russian Federal Security Service (FSB) and the KGB who fled to the UK where he was poisoned in 2006. The Skripals were also poisoned in Salisbury in 2018 in an action which also killed a UK citizen. These sorts of activities are unacceptable in someone else's country.
Countries can take extreme losses, yet continue fighting and survive and in some cases win their war. This 'special military operation' could go on for a lot longer yet.