Since you cannot directly take your certificates to HRD for verification without initial verification from the issuing authority, the time required for the HRD attestation process will take between 10 to 12 days. There are some factors that will affect the processing time
Any errors or incomplete application after submission can longer the process since re-submission will be needed.
If there are holidays due to festivals, bank holidays and others, the process will be delayed.
If the relevant authority is busy with processing and they get too many applications at the same time, it will cause a delay.
Each issuing authority has a different time and protocol for certificate verification.