When you try to rationalize the mechanics of the gunblade.

When you look at "dimetap elixir" and think "ONLY $10?!?!?! I NEED TO BUY 99 OF THESE!!!!!"

When you start confusing your windows keyboard shortcuts with FF menu shortcuts.

When your gameplay time reaches "FF:FF FF".

When your X button (or Square button in the case of FFVIII...) has been worn down to a smouldering crater.

When your first instinct while looking at swords is to wonder about their damage ratings and extra properties.

When you plow through a few bosses like butter before realizing you forgot to turn your TV on.

When you start coming up with "you know you play too much FF when..." lists based on things you've actually done.