When grounded, can an insulative material keep its charge for any measurable length of time? Or, I suppose, if it was a perfect insulator, would it discharge at all?
An example might be a charged rubber tire placed on the ground.
If it is a perfect insulator, then no charge can flow, and it will never discharge. Nothing is a 'perfect' insulator, so it's always a question of timescale - determined, primarily, by its resistivity. This is generally what happens during the build-up of 'static electricity'---for example, from rubbing together class and fur. Both of those materials are good insulators, and can hold a noticeable charge for an appreciable amount of time.