Note I'm not a teacher,
It's hight time for me to go to the dentists.
But I'm not sure about it. I think there is a slight difference in meaning.
It's about time / high time means that somebody (you) should have done something some time ago or a long time ago whereas I had better do sth sugest the necessity of doing it (going to the dentist) to avoid unpleasant consequence.
Interested in Language