you need your BHS stage 1, road rider, stage 2, stage 3, ptc (preliminary teaching certificate), child protection thingy, first aid thingy, then you can start teaching and get a log of your teaching hours, (between 300-500hrs depending where you teach/freelance etc) then you can submit it all and do your AI...you are then an Assistant Instructor. (then you can go further up and do your stage 4, then intermediate instructor, then become an Instructor) As far as how long it takes, they say it takes about a year to train for each stage but if you are already very experienced in riding and stable management you could do all the exams fairly quickly providing you know the 'BHS way' then do your teaching hours. Alternatively you can go through the ABRS route, but i don't know anything about that or you can do it through the RDA but don't know about that either
There are quite a few threads on this on the careers and education section.
You need your stages 1,2 and 3, and prelim teaching. You can take your PT after the stage 2, and teach after that, but won't be an AI.
How long it takes depends greatly on how you are - how you ride and what experience you have... Your best way of determining this would be to have an assessment lesson from an instructor at an exam centre/exam training centre.
The exams have just/are about to change a bit - I'm not clued up on it.
You can also go down the sports coach route and do your levels 1, 2 and 3
You can teach/ coach on your level 2.
You also need your disclosure, first aid at work or equine specific first aid cert and child protection cert. To obtain level 2, you need to have your level 1, then you will need to do a portfolio of 4 linked lesson plans for up to 4 riders be assessed teaching your linked lesson plans by your mentor,then assessed and passed by an external examiner coaching a session from your portfolio.
It took me about a year and a half to become an AI. Stages 1 & II are fairly straight forward and did these in about 6 months (Riding and Knowledge). Stage 3 and my PTT took about a year all in all.
PTT was one day a week training at the yard. Also did 3/4 days at the yard teaching my "guinea pig" pupils under instruction of my tutor which all counted towards my portfolio. I did my UKCC Level 2 coaching certificate alongside my PTT as the syllabus's are fairly similar.
BHS Website and their office are really helpful. Good luck!