When a lawyer sends and affidavit of assets before filing a civil case, what must the defendant do to comply ? How long do they have to get back to them with the affidavit ?
You have provided little information about the nature of the purported claim, the type of case that the lawyer threatens to file, etc., making it difficult for an attorney to deal with your question in this forum. However, if the attorney was writing you a letter stating that it was his client's intention to file such a suit, and that in the event you wished to stipulate to some type of payment plan or whatever, likely the affidavit regarding your assets which he is asking you to execute is a measure to determine what you might own which could possibly be used to satisfy a judgment for more than the amount of your insurance coverage. For example, if the civil suit is regarding a vehicular collision/accident, policy limits is often the best a plaintiff suing a defendant can hope for if the defendant owns nothing else but his homestead, which is exempt from claims of creditors. In such cases, sometimes the attorney will advise the plaintiff to accept offered policy limits from the insurance company insuring the defendant and settle for that rather than pursue more against a defendant who has nothing to take even if a judgment is rendered for an amount exceeding the insurance coverage available. Before a civil case is filed, there is no way to make you execute such an affidavit, because the rules of civil procedure regarding discovery do not yet apply to you since you have not been served with a summons or a copy of any complaint filed in the civil court system. However, it can sometimes be in your own best interest to cooperate in such efforts. You should consult an attorney before signing any forms sent to you by someone threatening to sue you because you may not fully understand the significance or legal effect of what you are signing otherwise.
There is not enough information in the question to give a great answer. If there is no case pending and a creditor is asking you for an affidavit of assets, it is up to you whether to provide that information. If it helps reach a mutually acceptable resolution, great. If it does not, they will use the information in any collection efforts against you. You would do well to consult with an attorney. If you cannot afford one, you can try Legal Aid.
Because there isn't much information here, I recommend consulting personally with an attorney in your area. You need to be sure that this is not a post-judgment fact information sheet that is required by the Court for you to complete. Failure to complete it could result in a finding of civil contempt.