Have you ever had an urge not to post?

Woman: “I have 14 children, Groucho”. Groucho: “You have 14 children? Why do you have so many kids?”
Woman: “Because I love my husband”.
Groucho: “I love my cigar too, but I take it out of my mouth every once in a while.”

If you know the Amp-Hours of your battery, I think you can approximate it like this (assuming that since the Radio has a 15A fuse, it's only running at a max 10A, including all amplifiers, etc):

Time_To_Dead (hours) = Amp_Hours/10

I'm guessing it'll be a LONG time before your radio runs down your battery. Looking at google lots of these batteries are around the 75 Amp Hours range. So going off of this you'll have 7.5 Hours.

Lots of assumptions here, but I'd feel safe guessing ~4 hours should be no problem. (I've applied engineering pessimism twice, but discounted any other systems that may need to be on to run the stereo ... which I don't think are much)