Beginner 35mm first attempt

Hi,

I have an LX90 telescope and a Canon EOS 10S 35mm camera with a 35 to 105mm zoom lens.I planned on pulling the zoom back all the way to 35mm.

I have seen some great 35mm shots of the Milky Way (I live in Australia).

I am going to a dark site next week and thought I'd piggy back my 35mm to a dovetail bar I have on the LX90.

So with maximum F 3.5 stop on the camera and Kodak 400 speed film, what sort of exposure time should I go for? About 20 minutes?


thanks,
Greg.