Setting an adequate response rate depends upon context.  In a single (one time) assessment where you are talking about a percent of a defined sample (e.g., percent of patients from a clinician’s practice), the percentage is less important than getting a representative sample and sufficient sample size.  Minimum sample size for one group summary statistics (not per subgroup) is roughly 40, but preferably 50.

In longitudinal studies of relatively healthy people who remain alive through the follow-up period, 80% is adequate, though 90% is better.  In longitudinal studies with sicker patients, 70% tends to be treated as adequate, but missing data is often not randomly missing which causes problems.