The Pex Piping itself should easily last 100 years or more but there's no 100% way to know because Pex Piping hasn't been in service for 100 years yet.
The brass fittings on the pipes will typically last 30 years from experience of replacing brass fitting in a 30 year old mobile home that had the old poly piping with the brass fittings attached to the pipe.
After 30 years the water tends to corrode the brass fittings out.
But I would say 30 years isn't too bad for the brass fittings to last. 30 years is a pretty long time to me.