Welcome to this forum.
You've touched on something that is a source of confusion for many people. As you say, even people who sell books about this stuff don't always seem to be sure what they're talking about.
In fact, you only need to visualise your goal once. As long as you do it with emotion and feeling, and completely expect to to receive what you ask for, and then let it go, your desire will manifest.
You must really want it (don't ask for what you think you should have or what you think other people think you should have).
You must believe completely that it is possible.
You must completely expect to get it.
When people say you should spend time visualising what you want two or three times a day (or whatever) they say so because it can help increase your desire, belief, and expectancy if they are not strong in the first place. For instance, many people want to manifest something but don't fully believe it could happen. Visualising it often makes it more familiar and, eventually, more believable. It conditions your subconscious mind, through repetition, to accept it as a real possibility.
The problem with visualising what you want several times a day is that you might do it with the attitude of trying making it happen. If you keep visualising in order to make it happen, it never will, because the thought behind your visualising is that it can't or might not happen (otherwise why visualise to make it happen). So you actually attract the thought behind it, which is it NOT happening.
Hope this is making sense.
The answer is, you only need to visualise once, as long as you are convinced it is possible and that you will get it.
If you have any doubts, then visualising it often can increase your belief and confidence, but don't do it with any thoughts or feelings of trying to make it happen.
The ultimate state to be in is a state of being. If you can truly be what it is you desire it will manifest easily and rapidly. Being in a state of being is what some people now call “being in the vortex”.
It takes practice to do this, and you have to get rid of any negative beliefs or doubts you may have.
Hope this helps.
Peace and love,