How long does it take for a foreign startup to launch in the US?

This will of course depend on the scope of your foreign / international operation. Are you trying to manufacture and sell t-shirts domestically, or are you trying to import controlled substances?

Forming an entity and setting up a bank account, even on behalf of a foreign entity, may be accomplished within a week under the right circumstances but simply doing that isn't exactly "entering" the U.S. market, so to speak.

I would defer to immigration counsel on the VISA-related matters, but generally your ballpark timeline, and costs and fees to be spent therein, will depend on more facts. I would recommend you speak with counsel in private to review those facts to get actionable advice.