There is no threshold that is tied to the amount of the grant used. Google does have a minimum click through rate of 5% for Ad Grant accounts, so a campaign/ad created and never touched again could lead to suspension because it wouldn’t be meeting their requirements when left dormant.
Is it useful in actually having an uptake in donations, or is it more useful for things like promoting an event, or like a walk-a-thon, or how is it best used by people?
I believe direct fundraising tends to be difficult with the grant. There's a lot of other non-profits competing for that same pool of keywords, and so I do think that tends to be tough, although it can be used successfully for donations. If you use it for donations, I would recommend that you have a killer landing page. And, what I mean by that is, when somebody clicks on an ad, and they get to your site, where you have content, it has to do a really good job of convincing and converting that person to become a donor.