also, with ABRSM, if you wish to take a practical exam above grade 5, you'll need to take their written theory exam at grade 5 first.
With Trinity Guildhall, there is no need to take a written theory paper.
As a guide (more for child learners, I think) for each year that you play, you should be able to progress by one grade. However you don't have to take all the grades in succession, and if you have previous musical knowledge, or lots of time for practice you can make much faster progress. I take my TG grade 6 jazz sax in 2 weeks time. I took grade 4 last year in June, I have been playing 2 years and 2 months (but I had previously played violin, trombone and recorder, but had never taken an exam). I haven't taken grades 1-3 or 5.
A teacher would be able to help you understand just what you need to actually do for all the peripheral work - not the 3 set pieces - the aural, sight-reading and scales in my case.