Trump is currently at 38% approval-57% disapproval in the Gallup presidential approval tracking poll, (although it has only been updated to include polling data since May 14). The poll is a three day average, so it reflects the Comey firing, but does not reflect last night's revelation that the President passed on classified information to Russian officials.
Because there is some portion of the public that will back Trump no matter what, there is a limit to how low he can go. But what is that floor? The lowest Trump has gone so far is 35%-59% on March 28. I suspect the floor is in the low-30s, at least until the next economic downturn. But maybe this revelation will be enough to push him into the upper-20s? We should know by the end of the week.