The amount of the offset will depend upon your LTD policy. You should not spend the retroactive SS benefit until you get this information resolved. There is likely an overpayment division for your LTD company that can calculate the amount of the offset once you receive your Notice of Award from SSA. The amount of your future LTD benefits may also be offset, depending on the policy.
The first step would be to get a copy of the policy. They should not have a problem with giving you a copy, particularly if you don't turn over the money until they give you a copy.
If the LTD policy states that there is an offset, it is your responsibility to notify the LTD carrier of your SS benefitss.