Does anyone knows how long it could take to replace an existing pacemaker / defib?

1 hour: If it is just a generator change (the leads are ok and don't need to be replaced) then it usually takes about an hour.

Pacemaker/ICD: Replacement of a pacemaker or icd usually only takes a few minutes, assuming the leads are working appropriately. The new device is placed in the same area where the old one was and the leads are attached to the new one. If the leads need to be changed then it becomes a more complicated procedure and takes longer.

< 1 hr: It takes 30-45 minutes of my time to do these so-called "generator changes." if you mean how long do the devices last between generator changes, the answer can vary from 5 to 13 years depending on a number of factors.