It was less than 2 months. Franken wasn’t sworn in until July and Kennedy died in August.
EDIT: it’s actually somewhere in the middle. Kennedy’s seat was held by Kirk, a Democratic appointee, from September through February 2010. However, I am fairly certain that Kennedy was basically unable to serve from March until his death in August.
Democrats basically had late September through early February to get anything done without a filibuster.
The size of the sample compared to the size of the population doesn’t really matter. What matters is how representative the sample is.
If they got a truly random sample (they didn’t), then 1000 is plenty.
Realistically, it’s probably 1000 Millennial and Gen Z Credit Karma users (already a niche group) who bothered to open an email and take a survey, which is not a realistic representation.