For me it was about 6 months until I started seeing any sort of significant results, and a full year for Kiddo to actually develop a proper hind end and topline.
It can take about 1-2 years to see a significant difference in the neck musculature, patience and consistency will be key once you've got them going correctly. The change will be small yet incremental as time goes on - suggest maybe taking some before and after pictures. Not only does the topline have to build up but his underneck muscles have to scale down. Remember in the stretch you want to be able to see the neck muscle that attaches into the shoulder start to bounce - that will indicate if you have him in an actively working stretch. The aim is to build a sort of tube through that muscle up to the poll.
It depwnds on so many factors his breed,age,nutrition,build,body wieght and mostly if he is being ridden so that he is working correctly and fluidly over his back and stepping under himself properly to engage those hind quarters and have a real swing in his walk and trot. Very often people have horses who r ridden too much in front and are “down and round” but not working through their spine and this will cause the topline to never build, my advice is try to ride the hind quarters more and let him have hos neck and with correct prompting he should work out what goes where
It took my horse 1 year to change shape. It can be quicker if you ride more than just twice a week like I did. It also depends on how you exercise him.
He was out of work for a while and his neck has made a slight change but it’s still upside down. We’re just now figuring out the stretchy trot and building his strength to be able to stay collected and on the bit during his walk/trot transitions. He’s been in this type of work for about 2 months now but i haven’t seen much of a difference and now I’m worried that either I’m not doing something correctly or its just going to take longer to see a difference
How long is a piece of string? It really depends on the horse, how willing he is, how long he was upside down, and why he was upside down.
Have you had him seen by a physio or massage person? When muscles contract up, it can be difficult (or in some cases impossible) to get them to release without some help. Lunging and pole work (including raised poles) will also help. There's a page on Facebook (Horsephysio) who puts up some really excellent pole exercises.
It can take months, and sometimes years to see a change, depending on various factors.