There are three avenues of relief that Congress has given innocent spouses. When to file the Innocent Spouse Relief form depends on the type of relief that you are seeking. If you are claiming that you did not know or have reason to suspect that there was an underpayment on your joint return or if you did not have actual knowledge of underpayment of tax and would like to have the erroneous return items allocated to the spouses who they belonged, you must file Form 8857 within 2 years from the first collection activity. Collection activity is not considered to be a Notice of Deficiency (90 Day Letter) nor is it a demand to pay the tax or a federal tax lien.
If you do not fit into the two types of relief above and are requesting relief because holding you responsible for the actions of your spouse would be inequitable, there is currently no requirement for when you must file Form 8857 (See IRS Notice 2011-70).