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Pierre, you will need a US federal cruising permit to bring the boat into the US. That's good for one year, not a minute longer. At that point the boat must leave the US for 15(?) days and you may apply for a new permit good for another year, which may or may not be granted.
Regarding state laws, I don't think you'll find any limits on your stays. For a US-documented vessel there are issues with registration and sales tax, sometimes in as little as 90 days, but for a foreign-flagged vessel that should not be an issue.
I would check directly with Customs, the USCG, and the Maryland state authorities to make certain of that though. All should have web site contacts, although of course sometimes it is still nicer to have a real letter to file in the ship's papers.