Do you mean just MTG? The one most people use is a leave-in oil, not a shampoo- and I'm unaware of the existence of a shampoo, though if there is, oops!
MTG noticably speeds up growth while it is being regularly used. I'm really sporadic in my use of it, sometimes I use it every other day and sometimes I use it once a month.
There have been times I noticed growth overnight from it. Try it on a spot where your horse has a nick if you can- where there's just closed skin and no hair yet. See if you see stubble the next day.
Here is a before and after of my TB- these are about 6 months apart. I kept his tail braided and used MTG as often as described above, of course also obviously giving him better nutrition than he had been getting.
I'm not sure what you mean in reference to retention of the growth- the rate of growth will not stay the same if that's what you mean, it only speeds up growth while it is regularly being used. The bit that grew will not weaken and break off, if that might be what you mean, but the tail does need to be properly cared for in general in order to gain and retain any growth, MTG or not. If you get a really nice tail grown out using MTG, you can stop its use but you have to baby the tail. The only thing MTG will really do will be to speed up growth, then maintaining that growth is up to you.
The TB pictured above was later sold, and now has a tail worse than in the before picture because it is left hanging freely and is rarely brushed, nothing put in it. A lot of horses do okay with that, but he naturally grows a bad tail, that's just him. I worked very hard to get it and keep it as full as it was.