If the granite is set at normal desk height, 30"; then a 36" wide drawer unit could be placed such that it is attached to both side cabinets and the rear wall. Now the granite will be supported on all four sides.

We spanned 24" for the dish washer, using 2" cleats on the rear and end walls. To clear span 36" with no support other than a cleat on the rear wall I think is pushing the limits. Having some steel bars let into the underside of the granite would do the trick however

If you do it like a table with stretcher bars mortised into the legs, you should be fine. No way I'd do what you're talking about otherwise, even with cleats supporting the rear. Just make it a small table, and you can even put a desk drawer into the stretcher, but make sure they are strong enough and actually supporting the granite.

I was told by two different fabricators it was three feet in the situation you're describing - cabinets on both sides.

Here is a link that might be useful: Another source

The real question you should ask yourself is can someone stand on it. Someday a teen or a painter or a housekeeper may put their weight on it. I know the granite can hold it self across 36", but realistically, you need to add enough support for a person to stand on it--because someday somebody will.

The thread is a decade old, but the subject is not. Stone, estone, solid surface, and other tops are veneer only. If you can't dance a Conga line across your substrate, it isn't strong enough.