Short answer. Ten days or more. Article 265 covers injuries not described in the more serious articles which incapacitate the offended party for labour for ten days or more, or require medical assistance for the same period. Below that threshold the injuries fall under the slight physical injuries provision instead.
What the law says
physical injuries not described in the preceding articles, but which shall incapacitate the offended party for labor for ten (10) days or more, or shall require medical assistance for the same period, shall be guilty of less serious physical injuries
Revised Penal Code, Article 265 — Less Serious Physical Injuries. Read the full provision →
Two ways to reach the threshold
Article 265 applies to physical injuries not described in the preceding articles, but which shall incapacitate the offended party for labor for ten (10) days or more, or shall require medical assistance for the same period. The conjunction is or, and that is the detail most often missed. Either measure will do. A person who kept working through an injury that needed treatment for the required period is within the article, and so is one who needed no treatment but could not work. The two are alternatives, not cumulative requirements.
It is a middle category, defined from both ends
The phrase not described in the preceding articles means this provision only takes what the more serious grades have not already claimed. So the analysis runs downward: first ask whether the injuries answer the description of serious physical injuries, and only if they do not does this article come into play. At the lower end, the ten-day mark is the floor. Fewer days of incapacity or treatment takes the case out of this article and into the slight physical injuries provision, which carries a lighter treatment of its own.
Two circumstances raise the grading
The article then adds two situations that make the same injuries worse in law. Where they were inflicted with manifest intent to insult or offend the injured person, or under circumstances adding ignominy to the offence, a fine is imposed in addition. And where the injuries were inflicted on the offender's parents, ascendants, guardians, curators, teachers, or persons of rank or in authority, a heavier penalty applies — subject to a proviso that in the case of persons in authority the deed must not amount to the separate crime of assault. Peso figures in the Code have been revised by later legislation, so read the amount from the current text.
The medical certificate is the document that decides it
Because the article is built on a number of days, the record that states that number carries the case. A certificate should say the period of incapacity for labour or the period during which medical assistance is required, in terms, and not merely describe the wounds. Get it early, from the facility that actually treated the injury. That number is not beyond dispute — the defence may contest it, and a court is not bound by it — but a contemporaneous certificate is far harder to displace than an assessment made months afterwards.