Tower Controller described a loss of runway separation event involving landing and departing aircraft; the reporter listing time away from the job as a possible contributing factor.

Narrative

I was Local Assist for our Local Controller who had taken the position after being off for several days straight. It was a busy hour and it had its technical moments. A C152 was cleared for take off from Runway 24 with a Cessna 172 on about a mile and a half final. The Conflict Alert (CA) alarm had gone off as the C152 was rolling for an aircraft in right crosswind to downwind. Our attention was removed from the aircraft on final momentarily. When Local noticed separation would not exist he quickly told the C172 to go around after the aircraft had just crossed the threshold; only he called him by the wrong call sign. The C172 never went around and ended up landing safely. Although the C172 had crossed the threshold before the C152 was 3;000 FT; from my vantage point it appeared as if the C152 had offset slightly north of the runway on a heading of 270. Biggest recommendation would be to ease someone back into Local after being off for 12 straight days. I feel it would have been far more beneficial had I worked Local and had our Local Controller work the assist position.