Tl;dr: Recommended dosage for people without prior nicotine use is 7 or 7.5mg per day, apparently. However, in another study, two patients administered 15mg per day and also got very good results (but with some transient side-effects).

In the study in question, they first give a case report on page 13:

"Continuous LDTN therapy (7 mg/24 h) was administered for 7 consecutive days (days 2–8)" to a female patient.

There is also this similar paper from 2023 by the same authors. In the section "Methods", they likewise report giving their patients dosages of 7.5mg per day. As they explain, they choose this low dosage because their patients were all "nicotine-naive", which I understand as a hint that in cases of (former) smokers, they might give a higher dosage.

Of one patient (41-year-old male, Case 3), they write that he mistakenly administered a higher dosage of 15mg/day, which he had to cease after 10 hours because of nausea. Yet, despite this (or maybe because of this?), this patient experienced a far-reaching recovery from the Long Covid symptoms over the following days.

Of another patient (52-year old male, Case 4), they write:

The nicotine-naïve and otherwise healthy patient agreed to a therapy trial using a nicotine patch (7.5 mg/24 h). Without consultation and contrary to our recommendations, the patient increased the nicotine dose to 15 mg/24 h on the third day of therapy; he then stopped the application completely on the fourth day after nearly complete symptom remission. He stated that he had not experienced any side effects of the nicotine application, which is why he doubted the efficacy and therefore applied two nicotine patches at 7.5 mg/24 h each starting on day three. ... In an interview three months after the intervention, the patient confirmed that he had not noticed any recurrence of the symptoms that had brought about his initial consultation.

Further up, somewhere in the introduction, they write something which makes it seem like that higher doses of nicotine are in principle safe and well-tolerated up until quite a large dosage, at least for patients with different diseases. (The mileage of Long Covid or ME/CFS sufferers could of course vary heavily here.):

These are all relevant occurrences of "mg" in these two papers.

Of note, all their patients administered the full dosage immediately from the first hour.