https://dgap.org/en/research/publications/preventing-next-war

With its imperial ambitions, Russia represents the greatest and most ­urgent threat to NATO countries. Once intensive fighting will have ended in Ukraine, the regime in Moscow may need as little as six to ten years to reconstitute its armed forces. Within that timeframe, Germany and NATO must enable their armed forces to deter and, if necessary, fight against Russia. Only then will they be in a position to reduce the risk of another war breaking out in Europe.

A few clarifications, from the comments below:

Europe:

Europe = the 27 EU countries + Norway, UK, Switzerland + The Balkans + Moldova (this is still open for discussion)

Not Europe = Belarus, a Russian ally - Transnistria, a Russian ally

Ukraine = is excluded, because it already is under Russian attack

Attack (also: offensive):

An aggressive projection of armed forces to occupy or recapture territory, gain an objective or achieve some larger strategic, operational, or tactical goal. Offensives can be waged on land, at sea, in the air or as part of a combined arms manoeuvre.

(So this has to have some critical mass. Not just a temporary, limited skirmish.)

What to do with "never":

It feels right to include "never" as a separate possibility. Russia may never attack, either because it's not their intention, it is above their capabilities, or there is sufficient deterrence. Resolution: When 2034 comes with no Russian attack, then both "2034 or later" and "never" will be accepted as correct. But it might be even more subtle:

• Russia has not attacked, but looks strong and aggressive - then I would recognize both "after" and "never" because you cannot tell the difference yet

• Russia has not attacked but looks weak and unwilling (for example: Chechnya has split off, Belarus has become friends with NATO, civil war etc.) - then I might recognize "never" only