Is studying several lectures with a شيخ considered sufficient to deem him a شيخ for the student, or does that require a longer association?

Whoever studied under a shaykh or acquired knowledge from him becomes one of his shaykhs, and this is not limited by a duration. The scholars said, "Whoever teaches you a single letter of what you need in religion is your father in religion." And Ali, may Allah be pleased with him, said, "I am a slave to whoever teaches me a single letter." Everyone who teaches you a verse, or a hadith, or a religious ruling is considered one of your shaykhs. For Al-Bukhari narrated only one hadith from his shaykh Hamdan.