He said it will happen "within the next several days". And now not not even bothering to cite US intelligence or the Pentagon (perhaps his own intel community has started distancing itself from the endless stream of false predictions?), the president said this is his "sense". According to Axios:

President Biden told reporters Thursday that his "sense" is that Russia will invade Ukraine "within the next several days," as troops and supplies continue to arrive at the border and international monitors report shelling across the line of contact in eastern Ukraine.

Calling the risk "very high", Biden explained that he sees "Every indication that we have is that they are prepared to go into Ukraine, attack Ukraine." CNN reported that when pressed for more details or evidence, he offered this:

Pressed on whether he believes an attack will happen, and if so, when, Biden said, "Yes. My sense is it will happen in the next several days."