I would say 6-12 months for a full-time developer, or several years for part-time developers. See details below.
There are about 2700 classes in VTK. All classes that interface with operating system or any of the 30+ third-party libraries, doing file operations, XML manipulation, console IO, render text, plots, widgets with labels, label mapper, display messages, interface with GUI, etc. are potentially impacted. There are 387 VTK file IO classes, so probably altogether about 500 files would need to be reviewed.
Developing the basic infrastructure - getting/developing converter classes for all platforms, taking care of vtkUnicodeString, Python interface, implement file IO on a few pilot classes, text rendering, and write tests for all these would take about 1-2 months for a full-time developer. In addition to this, you would need to spend about 10 minutes reviewing and fixing with each of the potentially impacted classes (500 classes in total), which would take about about 3-4 months for a full-time developer. In total, it would be about 6 months for a full-time developer. I’m usually too optimistic in workload estimations, so probably 12 months for a single full-time developer would be more realistic. If it is done by many part-time volunteer developers then it could take several years to get to all classes (and most probably classes that people rarely use or too complicated would never be reviewed/updated).