You need to evaluate him now, always now.  Players go on tilt.  They come off tilt.  They can tilt for long periods, or for one bet.  They win pots and feel good.  They lose pots and feel bad.  They improve.  They tire.  They change.  You change.  If you have an inflexible image in your mind of an opponent, then whenever he changes, your evaluation of him will be wrong.  If you have an inflexible image in your mind of yourself, then whenever you change, your evaluation of yourself will be wrong.  For up-to-date evaluation, there is no time but the present.