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.
Earlier, I mentioned that working on structure is Step One in developing IP, and is a precursor to making an aiki body. The first step of that process is learning how to align the joints so that you are not using dedicated muscle to hold your structure together, and so that force can easily be transferred in a clean "path" from point-of-contact to the ground, and back. Then, you learn to feel and manipulate cross-body connections from foot to opposite shoulder, and to manipulate tanden (dantien), meimon (mingmen) and kwa (not sure what the Japanese term is for that - the femoral joint and inguinal fold area) to receive/absorb and feed/project force both from within and that which is provided by a partner. These are not "stoopid jin tricks," but a methodological approach of systematic training in increments to condition and build an IP/aiki body.
Any authentic internal system has some kind of "dynamic stationary" standing exercise to work the foundational structure and connections, whether it's called zhan zhuang, tenchijin or 13 Points, or whatever. YMMV with these, as not all such exercises are created equal depending on the individual teacher, school and lineage; however, the general intention is to work the structure and connections. And as I stated, I have seen people start to develop structure within months, and be able to absorb a push and to neutralize a pull too. On an elementary level, certainly, with low-level stress from the training partner, but even so, it is the "stuff" of rudimentary skills and basic understanding, and will be built upon continually and forever. It should not take years if the person trains a little every day.
IMO and IME, that feeling of ease, balance and freedom of movement comes along after you've learned how to control your structure so that you are acutely aware of its position at any given time, and of any possibility of imbalance... and can adjust accordingly, without tensing up.