How long does it actually take to do grade 1 piano?

Unfortunately that can be answered by 'how long's a piece of string' I have had students do it in 2 terms, I have had others who have taken 4 years it depends on the amount of practice and commitment. I believe the average is somewhere in the region of 18 months.

(unless of course you mean the exam itself then it is 11 minutes!)

took me 2 years from scratch ie. not being able to read music at all.

Depends very much on the age and previous experience of the learner. As an average, a complete beginner beginning at around the age of 8 I reckon to be about 2 years. So a child who starts at the beginning of Year 4 would reach Grade 1 at the end of the Summer Term when they are in Year 5.

I have taught students who have been quicker and some who have been much slower. It does depend on the amount of work put in outside lessons and the general aptitude of the student. It is not a rush, a good foundation will reap benefits later on.

Don't get too hung up on how long it should take. Only do it when you are comfortable with the requirements, are at least reasonably confident you can do it and have reasonable experience of other grade 1 pieces and exercises. A broad range of experience is invaluable, will make you a better player and will help with your confidence on exam day. It will also help if something goes wrong. I found that focussing on exams only was a real grind after a while and my repertoire was very limited.