The right time to speak up about something that's wrong is always NOW. Why? Because if you knew and didn't say something at the earliest possible moment you will get blamed for inaction later.
If you don't say something when you see something is wrong, never say anything about it (at all). Otherwise you're asking for trouble later when the shit eventually hits the fan. "You knew and didn't say anything‽"
Even if someone gets upset at you for speaking up, that's still a better situation than being blamed later when the real finger pointing occurs.
"Don't look at me! I warned about this!" is a very real get-out-of-jail-free card in medium to large organizations. Especially if you have your objections in writing (save all emails!).
As a great example: At my work, the company made a piss poor decision to buy an (expensive) enterprise product that I warned would not work to solve the problem it was being purchased to solve. I warned them ahead of time that it wouldn't work. Then I warned them in the middle of the project and again, at implementation time.
When it didn't work, management came down HARD on everyone. In the middle of the finger pointing meeting I pulled up my emails which were sent to the people trying to point the proverbial finger and the meeting was over. Just like that! I saved the whole team with the simple act of voicing my objections in writing at every stage of the project.
If I didn't do that I have no doubt that some scapegoat would've been fired. Instead, no one got fired (sadly, because the normal rules of incompetence don't apply to the clueless/management layer, haha).
Unfortunately, to this day management never takes my offers of, "instead of purchasing this terrible 'enterprise' solution for millions of dollars, give ME that money and I'll produce a solution that's better in every way. I'll even have it up and running faster than we could requisition and install the product!"