I started my Employment on February 2017, with EU Blue Card in Germany. I want to switch jobs without re-authorization from the German Authorities. Can I do it now? (As it's been more than 18 months.) Or should I wait for it to be 2 years? (On February 2019)
You can change to a different company without going through the whole process, but you need to be aware of a few things.
Your new job must still be eligible for the § 19a visa. The job title should ideally be the same, or similar (i.e. something like Senor Software Developer and Software Developer or Project Manager and Technical Project Manager) is fine in my experience. If it's the same verbatim that makes it easier.
Your salary needs to still be high enough.
You have to go to Ausländerbehörde and tell them in person. If your title changed at all, they will give you a new accompanying paper. If it's literally the same, they will likely not give you a new one.
Keep in mind going there can take long. If you can, make an appointment. If you happen to be in Berlin, expect this to take a full day.
How long you've already worked has no bearing on this process. The Blue Card visa is not tied to a specific job, just the process of issuing it is tied to having a concrete job offer.
You can find information on the official website of the Bundesamt für Migration und Flüchtlinge. Here's the relevant part:
Any change of job during the first two years of employment requires the approval of the immigration authority (section 19a subs. 4 of the German Residence Act). Approval is subject to the same preconditions as initial issuance. If the minimum gross salary threshold has been raised during the intervening period, compliance with the new threshold must be proven. Holders of an EU Blue Card are obliged to inform the competent immigration authority if the employment for which the EU Blue Card was issued is terminated prematurely