It varies. None, 1 month or 2 months. It depends on the underwriter (the one who reviews the application documents that the loan processor passes on t them--and who you cannot talk directly to.)
In my case, they just wanted to see either a check from my IRA broker or a deposit hitting my checking account. I have never had to show 2 months.
It's all in the FNMA guidelines, which get updated periodically. It is easier now for retirees, since so many baby boomers are now retired and getting mortgages. The first time for me was in 2005 and Amerisave had never done one.
This topic provides information on documenting and qualifying a borrower’s income from sources other than wages and salaries, including:
selling-guide.fanniemae.com
"
Verification of Retirement, Government Annuity, and Pension Income
Document current receipt of the income, as verified by one or more of the following:
a statement from the organization providing the income,
a copy of retirement award letter or benefit statement,
a copy of financial or bank account statement,
a copy of signed federal income tax return,
an IRS W-2 form, or
an IRS 1099 form."
"If retirement income is paid in the form of a distribution from a 401(k), IRA, or Keogh retirement account, determine whether the income is expected to continue for at least three years after the date of the mortgage application. Eligible retirement account balances (from a 401(k), IRA, or Keogh) may be combined for the purpose of determining whether the three-year continuance requirement is met."
Note that it says "retirement account". That's why they don't accept a regular taxable brokerage account.
Also note that it is silent on how long you have to have been receiving the monthly distribution. Just "verified current receipt".