Given the speed at which mRNA vaccines can be designed, I'd expect there's already an Omicron-specific vaccine (and booster) in the pipeline. Factory production (going from tens of doses to tens of millions) is well understood, so it's just some studies, present results to a board, wait for FDA approval. But it could be quite a while yet.

First, there are safety studies - and after that, efficacy studies. There's no telling how long the latter might take as they're almost certainly not doing challenge studies (trying to infect people once they've been vaccinated).

First time 'round, vaccine efficacy testing went fairly fast - mostly because there was a lot of community spread, so enough infection in the control group to get a significant result happened quickly. There's no telling how long it might take this time, so... don't wait for an Omicron specific booster - get the one that's available now.