~5000 IU daily between February and May was barely enough to raise 25(OH) D level in my blood from 9 to 30 ng/ml.
Depending on who you ask, 30 is either the bound between "deficient" and "insufficient", or between "insufficient" and "sufficient". Regardless of who you ask, there's plenty of headroom until "excess".
Yes, it's possible but relatively hard to overdose on vitamin D, and due to a cock-up in some calculations in a study, until recently the recommended supplement amounts were about an order of magnitude too low: https://pmc.ncbi.nlm.nih.gov/articles/PMC5541280/
Depending on who you ask, 30 is either the bound between "deficient" and "insufficient", or between "insufficient" and "sufficient". Regardless of who you ask, there's plenty of headroom until "excess".