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.

Fission fragment engines are being looked at? That's cool, those are actually some of my favorite high-performance engine designs, because they can achieve efficiencies in the realm of fusion rockets but are far more feasible.

In essence, what they do is have an exposed nuclear reactor core, and funnel the radiation emitted by it through magnetic fields and use it directly for thrust. The reason for that being, the radiation moves at a very high speed, and specific impulse is directly proportional to how fast your exhaust is. Biggest downside is the extremely low thrust, but it can run for ages to build up speed.

The main challenges for building one is managing the heat to avoid a meltdown and explosion, since for optimal performance the reactor needs to be designed in a way that is difficult to keep cool, and you want it as powerful as possible while also using highly-enriched fuel (and an expendable coolant will just make it into a much less efficient nuclear thermal rocket and defeat the purpose)