If I understand you correctly, your opinion is that ① LNG will be much more expensive than Russian gas in the coming decades and ② Europe cannot reduce its gas usage enough to make that extra cost politically acceptable. Is that correct? If so, my answer to "how long" is "enough to depress the value of that pipeline considerably from its valuation as of two weeks ago".
LNG being much more expensive is pretty obvious - transportation costs are something like 90% of the final price. Pipelines are much more effective means of transportation for such things - build once, use for decades, at almost 0 marginal cost.
Reducing gas usage is entirely different topic, it would require to either: invest heavily in nuclear or: reopening of coal-powered plants.
Belgium (my country) imports about 400% of its domestic use in LNG, to then sell it to neighbouring countries via pipelines. Doesn't make sense if LNG was too expensive.
