For these sorts of scenarios I use this:
http://www.i-orp.com/You can use it to spitball allocations to tax-deferred, Roth, cash, other as well as play with inflation and investment returns. It's an easy way to see how long a given pot of assets will last under whatever scenario you want to test.
There's also FireCalc (
http://www.firecalc.com/) that lets you see how a stash would have held up across previous market periods. Although past performance doesn't mean much, at least you can get an idea whether your retirement assets would get you through something like the Great Depression.
You may already be aware of these, in which case, oops :)