Hello Vinodkumar,
Ideally, the relieving letter should be issued to the resigned associate immediately on his exit. The ideal practice should be to get the F&F settlement done on the last day of the associate's employment with the establishment and relieve him with a F&F settlement copy, relieving letter &/or experience letter (or combination of the both). Since the F&F settlement copy would have the associate's signature (as an acknowledgement), the establishment can pay out the dues to the associate within a couple of days (unless it's a termination case wherein you need to settle his dues immediately). In case the associate has to pay back to the establishment, he could be asked to submit a cheque on the last day of his employment, subsequently followed by issuance of the required relieving &/ experience letter. This will enable him/her to submit the same to his/her new employer.
But you need to have your HR process in place to get this process completed on time. Moreover, the "Mr/Ms Responsible" (for this activity) would have to be quite proactive and assertive to get the deliverables in time, since it would require a lot of coordination with the different departments/Finance-Accounts/etc. It's an excellent HR practice (since it would speak very high about the organization's excellence and professionalism) but it has to be driven from the top and that's where the role of the HR Head comes into play.
I know of few MNC's who religiously follow this standard process, but these are companies with excellent HR policies and practices. However, I have seen many many other organizations who either settle your account after a few days or even later to the associate's exit. In most of the Government (both state Govt and central Govt.) organizations, you would find the relieving/settlement process getting done days/weeks later.
Best Regards, A.B.