How long, is too long, for a respondent to have, whilst responding to
an accusation of workplace bullying?
Given the few details you have written, there's no way for anyone to say X is too long, but Y isn't.
This depends on local laws, and the practices of the recipient of your accusations. In some locales, there is a specific period of time during which you must get some sort of response. In others, there is no such requirement. For some investigating groups, they act immediately. For others, they can take months or just bury your accusations forever.
Ask whoever you sent your accusation to how long you should expect to wait for a formal response.