First, good for you for wanting to be prepared! You so seldom see people preparing for life after the military, and then they complain.
It's a tricky question, because there are so many variables.
With 22 years, I would think the two of you would be covered under TriCare for Life. Will that be the case? If so, I would look up the current annual premium, add about 10% to that, plus any current deductible (not sure if that applies, or how far you will be from a MTF). That will be a good starting point to determine how much you will need, especially if you can take care of a full year's premium and breathe a little easier about medical care. If he is able to find another job right away, with benefits, it may not be an issue, but better to be prepared.
Will you be moving back home? A GTC will have to be turned in prior to final out processing, and it will take several weeks to receive travel reimbursements that may be due. Your tickets and household goods will be paid for if you're flying from overseas, but if you plan on driving and/or doing a DIDY, you will need to have enough saved to cover that OOP, as they will typically not advance monies when a member is separating for any reason.
Are you in military or privatized housing? If privatized, they are paid at the end of the month, not the beginning, and are not authorized to deduct BAH from final paychecks. So you will need to have enough to pay them before out processing, as they will not bill you.
Also, be aware that a final paycheck can and will be held for up to 60 days (I was lucky, mine was only 2 weeks) while they double-check that you don't owe anyone affiliated with the military any money (Exchange card, library, Enlisted Club, etc). This is separate from any travel vouchers you will need to submit, and that money may also be held.
In short, I would recommend
a minimum of 6 months' expenses
at least 6 months' health premiums
enough to cover a full month in housing (plus any damages/fees you think you may incur)
enough to travel to your home of record or pay
any difference between shipping to your HOR and your chosen destination
Getting out of the military is quite the hassle, since it's almost impossible to have a job solidly lined up before you do. Best of luck to you and your DH!