Short answer. The number of days the victim could not work, or needed medical assistance, is what sorts the offence into a band. Ten days or more, without the graver consequences the earlier articles describe, makes it less serious physical injuries. Fewer days drops it to slight injuries; longer or permanent harm raises it.
What the law says
Any person who shall inflict upon another 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 and shall suffer the penalty of arresto mayor.
Revised Penal Code, Article 265 — Less Serious Physical Injuries. Read the full provision →
The ten-day line, and the two ways to cross it
The article sets one threshold and gives two alternative ways of meeting it. The injuries must either incapacitate the offended party for labor for ten (10) days or more, or require medical assistance for the same period. Either one is enough. So a wound that kept the victim off work for only a week can still fall in this band if treatment ran for ten days or longer, and the reverse is equally true for a labourer who could not work for two weeks even though the doctor discharged him earlier. This is why the medical certificate, and the number of days of healing or attendance it states, so often decides which charge a prosecutor files.
How the bands fit together
Article 265 is deliberately residual: it catches injuries not described in the preceding articles. The graver article ahead of it covers consequences such as insanity, loss of a limb, blindness, incapacity for the victim's habitual work, deformity, or illness or incapacity running past the longer periods that provision fixes. Below Article 265 sits slight physical injuries, which covers harm needing fewer than ten days of attendance or incapacity, and ill-treatment causing no injury at all. Nothing here applies where the offender intended to kill — that is charged as a form of homicide, whatever the healing period turned out to be. Numbers of days sort the offence; they do not settle intent.
The current penalties, and why the peso figure matters
The base penalty is arresto mayor. Two things raise it. Where the injuries were inflicted with the manifest intent to insult or offend the injured person, or under circumstances adding ignominy to the offense, a fine of up to fifty thousand pesos is imposed on top. That figure is current because the article was amended by Republic Act No. 10951 (2017), which revised the money amounts throughout the Revised Penal Code. Many printed and online copies still show the 1930 figures, so check the version of any text you rely on. Where the victim is a parent, ascendant, guardian, curator, teacher or person of rank or authority, the penalty rises to prisión correccional in its minimum and medium periods instead.
What to do about the evidence
Because the classification turns on documented days, the paperwork is the case. Get examined promptly, keep every certificate, prescription and receipt, and make sure the physician records both the healing period and the period of medical attendance rather than only one. Follow-up consultations count and are easily left out. Photographs taken over successive days, and any employer record showing days absent, support the incapacity limb. Note also that where the victim is a person in authority the article steps aside if the act amounts to the separate crime of assault. Whether you are the complainant or the accused, the band you fall into is arguable, and it is worth taking advice before any statement is signed.