Depends on the system. Classical fantasy adventuring? Most if not all sessions. Adventure and Sword&Sorcery? Sometimes, half perhaps. Character drama? Very seldom.

I look at how the system spends its page budget and use that as a guideline. If there is a chapter for combat, one for harm and recovery and one for combat magic then the system wants me to focus on those parts. Also I look at how the players/characters are rewarded and try to have each session hit several of those criteria. So if the only (reliable, non gm-fiat) way to earn rewards if through combat then you bet your sweet ass there will combats each session.