Many have asked about Diksha mantras vs non diksha mantras.
Since I have some experience of both, let me elaborate:

With non Diksha mantras what happens:
Initially, the mantra is being done and no real discernible effect is being produced in the consciousness, surroundings or your life.
(Perhaps one picked up the mantra from a book... etc)

Depending on what anusthana you are doing and what number, literally nothing happens. This is especially TRUE if Brahmacharya is non existent..
Nothing happens for days or even months.

What I said is not completely true. Nuances are there.
Let me explain:
On the other hand, if Brahmacharya is good, you have experience of other anusthanas and your Niyama is good, if around 40 or 50 days or double that (based on 2-3 hour sitting).. something starts occurring..

So, while it is true that non Diksha mantra will not produce an effect straight away, it must be said that if one perseveres and keeps going after that with competence, at some stage the effort of sadhana is good enough to burn the karmas necessary to cause some knowledge.

Please remember that for certain Devatas, a fixed number of years is necessary to get anywhere in sadhana... (at 2-3 hrs a day).. according to the texts itself.
So... this is as far as non Diksha mantras go.

Diksha mantras also require effort but the effort for internal realisation, depending on the sadhaka, varies:
For one it may still be 10 years, for another or may be a few years only (as minimum time never varies in text)
So one has to put in serious effort... very serious!

The idea that Diksha mantra produces instant realisation is true in exceptional cases ... examples of great saints who did it are there, but since they are not around, what will you do but persevere and check progress:
Knowledge level/material level/release from obstacles/shanti

Majority of the time check if there is a exponential change in:
Knowledge level/
material level/
release from obstacles/
shanti/internal bliss
If none have occurred, try again for longer. It’s not a race anyway.