I was looking through the XML files and found that based on quality, the Flame Fruit had a limited lifespan.

Horrifying/putrid - 1 sim hour
Foul - 2 hours
Bad - 4 hours
Neutral - 8 hours
Nice - 16 hours
Very Nice - 24 hours
Great - 32 hours
Excellent - 40 hours
Outstanding - 54 hours

Perfect flame fruits do not lose their benefits.