How long should an international bank transfer sent via SWIFT take, and why do some transfers take 3–5 days instead of the expected 1–2 days?

Bank to Bank transfer is usually 1-2 days (keeping into consideration time differences, if applicable). Also you need to possibly understand if a correspondent bank is involved. All in all your money usually would be available on Day 3. However, banks might keep this money for a day or two, to earn some float on it.