Question about extra time arrangement (GCSE)

I don’t know if this is a stupid question but I get extra time, I sometimes use it but I sometimes don’t (in mocks). When I use my extra time, do I have to stay for the whole duration of the extra time or can I leave whenever I want during the extra time window? I’ve only used it once during my mocks and I just walked out halfway during the extra time and the invigilator just escorted me out. Is this the right thing to do? I was never told explicitly. Do I have to tell the invigilator I’m finished? I’d really appreciate if someone can answer this, thank you.