My player group are heading for Ziost as they need a off the chart place for reasons. All life on Ziost was killed in 3637BBY (as seen in the Old Republic MMO) and Wookiee says the planet stays depopulated.

Obviously after a time the power will die, and even the droids will stop moving, so my question is how long do Old Republic buildings and stuff last? On Earth our Skyscrapers would be gone after about 200-250 years, and brick buildings not much after- what about Star Wars tech ones though?

Also what tech might survive? Again on Earth aluminum, honey, glass, plastic can last centuries- what Old Republic tech would have survived 3638 years on a desolate, cold world like Ziost?

Ancient stone structures would likely have survived, so most Sith tombs, if you look at the pyramids of Egypt and other ancient structures.

As for modern structures, many SW materials are suggested to be far stronger and more resilient than our Earth materials, so it is possible the shells of many buildings could have weathered the years, but are likely to mostly be in a fairly poor state of repair.

Structures designed to withstand punishment, such as shelters and military bunkers could potentially have lasted mostly intact. It is also possible they could still have power, as SW power sources seem far superior and even hint at endless energy (also depending of fuel type), especially with the minimal drain due to no inhabitants.

There could even still be working droids, especially any that went into a standby mode until they detected any new life or changes on the planet. In fact, if there were droid factories that produced droids, these could have continued quite happily while resources were available. If the droids were smart enough, they may have used these factories to constantly facilitate repairs over the centuries and even scavenge the cities, failing droids and land for materials. As time went on, more droids would fail and resources would become minimal, but a good number could still be functioning, maybe with their own sentient view of the universe and their place in it. These droids could even have their own form of governance and opinions of non-machine life.

Essentially, in the SW universe i think there is a far greater chance to find actual "empty" cities still standing than you would on Earth. It is also likely that by this time, plant growth may have finally come back, though given the nature of the "life killing" event that took place, it is possible it killed off this possibility. Of course over time foreign bodies could have brought new life to the planet.

Given the almost magical nature of some events and settings in the SW universe, Zoist could be in almost any state you want it to be, just a planet with no sentient life (except maybe a bunch of Sith alchemy beasts and "evolved" droids).

One question, how does the Wookiee know this?

It is likely only legend that Zoist stays depopulated, and we all know that legends are not always exactly what actually happened, so change it up to add some mystery.

Waiting droid armies, crazed force creatures and hidden dark Jedi are all things that could be fun to introduce.

What do you want to have lasted all those years? What do you want to be there?

Make it so.