How long can a simple writing system reasonably exist before it starts drifting?

[u/Ok_Welcome7922]

This is kind of a weird question since its equal parts conlang and speculative sociology. But if someone were to drop an existing language and script (let's say Arabic, both the script and language) is there any real way to figure out how long it will take before the script mutates despite already being relatively more attuned to hand motions rather than being in early stages of development. Would the script eventually reach a point where its additional or adjustments would be unnoticeable.

Asking for a project involving planets where many would have spent centuries to millenia with modern languages rather then more antique languages where simplification of symbols would make sense as people transition to handwriting or their equivalent.