Drawing 5 specific cards from 40 (no replacement):
P = (35 choose 0) × (5 choose 5) / (40 choose 5) = 1 / 658,008
≈ 0.000152%. At 1 game per minute, ~457 days of nonstop drawing for one expected Exodia hand.
In a 60-card deck, odds drop to 1 / 5,461,512. That's why competitive Exodia decks run the 40-minimum.