If the batteries are fully-charged, then they should probably do fine. The question is whether they're AGM, GEL or "maintenance free" wet-cell batteries. If they're wet-cell batteries, then they will probably need to be re-charged occasionally while the boat is on the hard.
Detaching the negative means that there won't be anything using the battery, discharging it. However, all rechargeable batteries self-discharge to some degree, with wet-cell batteries having the highest self-discharge rate of the three commonly used lead-acid types.
As for how long the batteries will last, really depends on how well they're treated. Most are killed by neglect far sooner than they would ever die.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.
—Cpt. Mal Reynolds, Serenity (edited)
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Still—DON'T READ THAT POST AGAIN.