I'm kinda confused about the use case here.

Once someone is dead, the window to graft in an artificial heart is waay smaller than the procedure would be. So the only time it would be useful is if the person were already "on the table" so to speak.

Think about it like this. Any situation in which the biological heart in the person would need replacing, the switcheroo needs to be done under bypass. You have to hook everything up to keep blood (and the oxygen it carries) flowing. No bypass = cell death across the body. Once enough cell death occurs, ain't no coming back.

Now, there would be niche cases where you already have the chest open that an artificial heart could be placed, but you'd have to have one just sitting around unused. Which isn't common.

So you can't bring someone back as a resurrection. Like, they're declared dead, you can't just crack the chest and hook the machine up and expect and everything else to function. Not impossible, but the process of death ramps up fast once cells start shutting down, and that takes a matter of minutes in some cases. Just opening up the chest cavity can take 5 to 10 minutes, depending on situation and the people involved. Iirc, just getting things connected for bypass takes a lot of prep, but maybe you could be doing that at the same time as cracking the chest open.

But you'd still need to work faster than the cascade of everything shutting down, and I am dubious in the extreme of that happening outside of an OR where they are already prepped for cardiac surgery.

With all that said, assuming you did get things hooked up before cell death started fucking things up, you could expect a fairly decent amount of time. Days for certain since you'd already be in hospital. Weeks probably, since once you get past the first few days, the major risks post surgery decrease.

Months isn't absurd. Artificial hearts can already give people months, so it's within reason that your hypothetical patient would be able to.

But years? No way. Even with top notch medical care, the added stresses on other organs would be too much for someone in that condition. I just looked up the record for living with a full artificial heart. 555 days.

So I suspect that your brain dead patient would be gone well before that.