You keep using this meaningless word that is mediation, as if it is a good idea... whereas nobody knows what you mean by that.
if you mean bhavana, then say bhavana and the answer to your question is obvious. Bhavana means you do it as much as you can, each day, all the time.
If you want to be more specific, then you use jhana, sati sampajanna or the other 2 Samādhibhāvanās https://www.buddha-vacana.org/sutta/anguttara/04/an04-041.html
Sati sampajanna is obvious, since you do that as long as there is vitakka, vedana and sanna, so again pretty much all the time, certainly not 15 minutes a day, and then spending 23h45min not developing thoughts renunciation, non-ill will and harmlessness.
you will never clean the mind by spending 23h45min each day not developing renunciation, non-ill will and harmlessness, by not forcing seeing things as anicca, dukkha anatta.
Development means exactly the opposite of whatever you are doing in 15 minutes per day, which is certainly not training the mind.
Even the people who train and clean the body do not claim they train and clean the body by spending 15minutes per day on the body, and the mind is even harder to train and clean...
Jhana is when you want sukha and you cannot do that all the time, you can pause for a few hours up to a few days, especially when you are not a idle bikkhu.
The last 2 Samādhibhāvanās are done when you want to acquire knowledge of karma and nibanna, so it is not relevant for you here.