The DPR has been implemented since 1998.

GDPR had been announced 2012, implemented fully in 2016. Active enforcement will start May 2018 with again a temporary period to allow companies to correct. Refusal to comply after that can result in penalties up to a maximum of 4% of the companies global revenue.

How much courtesy lead time does a company actually need to comply?

That's not how the law works in most European countries. Class actions don't exist, so the usual strategy is to sue the biggest company or greatest offender, since that results most likely in the best defence or the best case.

The summary of the court of the case, if ruled in favor of the one suing or in favor of the public interest, will be used to prosecute all other offenders if they do not comply. If the defense wins, it can be used by others as a defense.

While not 'fair' it works as the smaller fish will probably go bottoms up trying to mount a proper defense against larger governmental or lobbying groups which results in a no-win scenario for all: The company is dead and there is still no ruling, or a ruling lacking proper defense.

What, if you get screwed by Google do you ue all the companies or only Google?

Or say Intel users that are now sewing on the meltdown bug should they get involved in AMD too from some feeling of solidarity?

In this case someone did something illegal and someone else complained to the justice, should they first find all (I hope you understand what all means, aka don't forget anybody) and try to do what? start 1000 processes in justice? It makes sense to start with the bigger criminals, if the court decides favorably then you continue to the next ones.

1. Why should their be a "courtesy heads up"? Most of these defendants know they're breaking the law. The regulatory agencies have made their interpretations of the statutes known.

2. Do you realize how much manpower it would take to require that all separate cases be tried at once? You might as well just come out and say you don't want any cases to be tried at all, as that would be the outcome.