Hi Cosmos,
Meditation with a mantra is more powerful than many other types of meditation. This is because we are not only bringing our mind to stillness, but the mantra itself has an energetic vibration that purifies the subtle nervous system. For each individual, there is only a certain amount of purification that they can experience each day, without running into issues.
Experienced practitioners could increase the timing for Deep Meditation up to 25 minutes per session, or even 30 minutes. Thirty minutes is considered the maximum amount, even for advanced practitioners. So, that is 30 minutes, twice each day. 20 minutes is considered optimal for most people, because it produces a good amount of purification, without people running into overload issues.
At the weekend, if someone has no work, or care duties, then they can increase the number of sessions that they are practicing each day by doubling-up on the morning practice session. So, the regular morning practice is done once, with a rest period at the end, followed-by a short break, and is then done again. If people have longer off work, say a whole week, they could also double-up on the evening practice sessions, so they would be doing four sessions a day. They would go to three sessions on the first day off, then to four a day, and on the last day off go back to three again.
And yes, there are many experienced practitioners who have done this without running into issues. The key is to only make changes gradually, over time, rather than suddenly changing practice timings.
There is more on this here:
There is also an AYP practice chart which can be downloaded for free from the AYP Academy site, which shows the various AYP practices and the timings that they can be safely increased by for people who are stable with their existing practice. That can be downloaded as a PDF here: