Less than two months from the mousetrap (and Hamlet's departure for England) to Laertes' return.

While conscripting Laertes to murder Hamlet, Claudius reveals this: (4.7.78-80, 89)

"You have been talked of since your travel much,/And that in Hamlet’s hearing,…/…not two months since,…"

So Hamlet can not have been gone on his sea voyage more than two months.