I would say as much as you can get! You will be surprised that most private gigs need to have the assurance that YOU are going to show up.
Some corporate gigs have paid us 100% up front and weddings must be at least 50% and certainly cash any cheques immediately.
I have kept deposit monies back from the band (I am their manager), until the gig is completed however.
"Life is what happens while you are busy making other plans" - John Lennon