In Japan, there were 9 BDs released for season 1, each of which had one episode of Sword Art Offline on it. Likewise, there were 9 BDs for season 2, and one episode of "Offline" on each. (Obviously, Extra Edition goes with its corresponding "Offline" episode.)
It makes sense that you would watch a given "Offline" episode after watching all the "Online" episodes that came on the same disc. Here are the correspondences:
Season 1
╔════╦══════════╗
║ BD ║ Episodes ║
╠════╬══════════╣
║ 1 ║ 1 - 2 ║
║ 2 ║ 3 - 5 ║
║ 3 ║ 6 - 8 ║
║ 4 ║ 9 - 11 ║
║ 5 ║ 12 - 14 ║
║ 6 ║ 15 - 16 ║
║ 7 ║ 17 - 19 ║
║ 8 ║ 20 - 22 ║
║ 9 ║ 23 - 25 ║
╚════╩══════════╝
Season 2
╔════╦══════════╗
║ BD ║ Episodes ║
╠════╬══════════╣
║ 1 ║ 1 - 3 ║
║ 2 ║ 4 - 6 ║
║ 3 ║ 7 - 9 ║
║ 4 ║ 10 - 12 ║
║ 5 ║ 13 - 14 ║ (and also the recap episode)
║ 6 ║ 15 - 17 ║
║ 7 ║ 18 - 20 ║
║ 8 ║ 21 - 22 ║
║ 9 ║ 23 - 24 ║
╚════╩══════════╝