IME, if you get good instruction and feedback, it should -not- take many years to develop basic or rudimentary skills. I've seen people start to be able to "do stuff" within 6 months, under the right teacher.
Yes, you have to practice, and practice mindfully. And, there are infinite layers of skill that we'll spend the rest of our lives striving for as we refine and build on what we have. But that is not the same thing as being given the tools at the outset to start building your foundation. If it took years and years to get a foundation in, we'd all be living in grass huts.
In my experience it could take a few years to even get some basic skills -- unless you're a training animal unlike us half-assing it
. If it's "stupid jin tricks", sure, a few months or even weeks could create enough mind-body connection. But it mostly depends on what a particular teacher considers base skills. For example, if the aim is IP, hara/dantian development has to take place, and manipulating that stuff takes years and years. If the model involves spiraling, that's going to take a long time to manifest in waza and even more time in high-pressure stuff like sparring and fighting. Some teacher might think connection/dantian/spiraling is all fundamental, so the dividends are going to take some time.
Even after getting some rudimentary skills, it will not be very convincing to an observer as Lee's excellent post has said above. I will add though, that one thing that the observer cannot fully measure, is how the IP player feels inside, like the level of ease, balance, and freedom of movement they feel when performing a certain task. In early development I personally might not be a compelling case when trying to have an observer feel the difference of IP vs no-IP (my own personal fault and nothing to do with the material or teacher). But the way I
feel pre-IP and post-IP training is certainly a paradigm shift for me. For example performing a demanding aikido or weapons routine, and comparing what it felt like before and after IP training, and realizing that IP allows me to perform the same routine with only a fraction of the energy, better balance, and better external effect (again, not too noticeable from the outset at first, but a vast difference on how I feel). This aspect of progress is even harder if not impossible to convey in written form.