I have wondered this for a while and now Dave Ramsey has a cool online tool to calculate for you.
enter your AFTER TAX monthly take home pay:
heres an estimate depending on tax bracket assuming 28pct taken out in taxes.

years salary * .72 = after tax yearly money
post tax yearly money/12 = monthly after tax money (this is what you type into Dave’s Gazelle tool)

http://www.daveramsey.com/mobile/gzbudget/