My kids and I began on the Island in February and have been playing a new map every month or so. So far we have played (in this order) the Island, Ragnarok, Extinction, Aberration, SE and we are now on the Center. Before hopping into the Center last weekend we went back to the Island to defeat the Tek Cave, only to find that our buildings were all still there - from March! The dinos were dead but everything else held.
I had thought the saves wouldn't last or the buildings would degrade. Any idea how long they last? These were stone structures for the most part, if that matters. Also, we use the routine Obelisk/Transmitter transport out/in, if that matters as well.