There isn't a fixed time period. It happens when the agency is ready.
Sometimes, it will be adopted as proposed. Sometimes it will be tweaked with concerns raised in public comment. Sometimes it will be abandoned entirely. Sometimes it will be revised significantly in another direction and there will be more public comment. The more comments there are, the more time it takes an agency to analyze and consider those comments.
Agencies don't propose regulations unless they have a desire (often, a need) to get something in place. After all, the agency proposed the regulation in the first place. So, the law doesn't worry too much about delay in adopting a final regulation.
It isn't uncommon for tax regulations to be proposed and then languish for many years, if new administrations lose interest in them, or if there are serious concerns that the proposed regulations may be flawed in some circumstances or open up a new tax loophole that wasn't anticipated and it isn't easy to draft a solution to the problem.
Frequently, regulations are adopted as soon as possible.
Most often, a long delay in adopting a proposed regulation reflects some sort of high level political decision by political appointees in the agency to pause the regulatory process while the administration decides what it wants to do.