If it's a car showroom then I would go for monthly.
One other problem you sometimes get with car showrooms, we do a large Caffyns plus a smaller showroom, is car wax on the glass from when they clean the cars. I'm not quite sure how it gets there but it does.
I'd have a close look at the glass first and if there seems to be odd marks almost looking like they're etched into the glass then I'd point it out and up the price a bit as the only thing I've found that removes it is neat vinegar on a mop and then wfp in the normal way.
Me, minus any wax on the glass, I'd go for £35 a month and sell the quality of work.
Cheers