Well, it depends on just how much you put in the reactor. The half-life is millions of years, so it's not really going to decay on its own (that is, you have to have the reactor turned on for it to do stuff, using its equivalent of control rods) meaning you can treat it basically like any other fuel.

And for how much you can get out of that fuel, it is theoretically to our best ion drives what those best ion drives are to average chemical rockets - so think running for months or years on end and capable of eventually reaching hundreds of kilometers per second.