When does the lawsuit happen...?
Reading the comments from the companies and industry groups, I'm going to opine that they don't plan to sue for declaratory judgment, as it's more useful to have a bogeyman law on the books where it doesn't matter -- the whole population of Montana is about the same as that of the Rochester, NY metro area! If Montana actually does anything, I'd expect the victims of that government action to get significant resources to defend themselves, and kill the ludicrous and obviously unconstitutional bill that way.
By contrast, if Congress did the same thing, I'd expect an instant application for an injunction!
Of course, the whole point of this law is a combination of virtue signaling and pushback against marketing from TikTok, i.e. to create a larger fuzzy boundary disfavoring certain types or vehicles of speech.
(There is, of course, a possibility that Apple/Google/California might sue Montana for demanding they limit access to an app, but again I suspect that will wait until Montana tries to do anything about it... the people in Montana that would have to initiate proceedings against Apple/Google are almost certainly less partisan and/or raving loony, so may decline to prosecute the unprosecutable).