Andrey Piontkovsky, in a recent interview, reckoned within half an hour of a nuclear weapon being used by Russia, NATO forces could take out the entirety of Russia's Black Sea fleet, and within a few days take out all Russians in Ukraine who have weapons.
That rather depends on where they're planning to hit. You're assuming a front-line position, which is not necessarily what they'll do, as the frontlines are mostly in the four territories they're trying to annex. If it's a distance back from the front line then they could probably do it with little warning. Given that they have aircraft that can carry such munitions, neither Ukraine nor the West might know until after the fact. There might not be the obvious sign of preparation that there is with the likes of ICBMs.
Bear in mind that Russia have nukes with a wide range of yields, from the 10's to the 1,000's of tonne. The Texas explosion in 2013 was c.10 tonnes, and the warehouse blast that rocked Beirut in 2020 was around 300-400 tonnes equivalent. So what if they use one of their diddy little nukes, say 20 tonnes (if they have any that small)... is that sufficient for NATO to get involved the same way as if it was, say, a 1 kt nuke? I mean, is the red line the fact that it's a nuke at all, or is it the size, and the effect?