How many days does ACS allow for submitting missing documents?

Within how many days we have to submit our requested documents to ACS assessment?

I have asked to submit the work experience letter from current employer and it may take time to get it.

ACS assessment allows 30 days to upload the missing documents

If not uploaded within 30 days, they will evaluate your application without the document that they have requested.

That would probably be your loss. So, i would suggest to upload it as early as possible.
Also, do not wait for 30 days to finish.

See this sample email from ACS and it clearly mentions 30 days.

acs-assessment-australia-upload-documents-email-truvisa

The faster you upload, the faster your get the ACS assessment result for submitting your Australia skillselect Expression of Interest (EOI) application.