There are a number of factors that have to be considered, including, but not limited to, your own voice projection, mic quality, EQ, FX, etc. ...
I am usually about 2 - 3 inches away, but will 'back off' when hitting a 'big' note ... I will also get closer for effect ...
And don't be fooled by some of these big name stars when they are hitting a big or high note and the mic is arm's distance away ... While some of them have that power, others have a sound engineer adjusting the volume at the same time they are hitting the note ...
Just make sure that whether you are singing or speaking you can be heard and very clearly understood ... Being able to turn off or lower effects(reverb/delay) when you are about to speak over the mic is a plus...
t.