tl;dr:

• 1 confirmation: sufficient for small payments less than $1,000.
• 3 confirmations: for payments $1,000 - $10,000. Most exchanges require 3 confirmations for deposits.
• 6 confirmations: good for large payments between $10,000 - $1,000,000. Six is standard for most transactions to be considered secure.
• 10 confirmations: suggested for large payments greater than $1,000,000.