user: Question: How many weeks has the opponent been san francisco 49ers?
Columns: ['Week', 'Date', 'TV Time', 'Opponent', 'Result']
Types: ['real', 'text', 'text', 'text', 'text']
Sample row: [1.0, 'September 6, 1998', 'CBS 4:00 pm ET', 'at San Diego Chargers', 'L 16–14']
SQL:

assistant: SELECT COUNT("Week") FROM "Table" WHERE "Opponent" = 'san francisco 49ers';