According to the letter written to his sister (in the closing chapter of Frankenstein), it would seem that the monster arrives upon Walton's ship as soon as Victor speaks his last words. Walton describes Victor's last moments in the following way:
His voice became fainter as he spoke; and at length, exhausted by his effort, he sunk into silence. About half an hour afterwards he attempted again to speak, but was unable; he pressed my hand feebly, and his eyes closed for ever, while the irradiationof a gentle smile passed away from his lips.
Curiously, one may state that the monster does not arrive as soon as Victor dies (given Walton is writing to his sister when interrupted by "a sound as of a human voice, but hoarser; it comes from the cabin where the remains of Frankenstein still lie"). Instead, it could be supported that the monster arrives well after Victor's death as Walton is writing about what has happened with Victor. As for the actual textual support, the last words pass Victor's lips and two paragraphs later Walton hears the creature.
Therefore, one could look at the writing as a clue to the creature's arrival and state that the creature arrives well after Victor's death, or one could look at Walton's letter alone and see that the creature's arrival had to coincide with Victor's death.