According to Spongebob in the Spongebob Squarepants Movie, Spongebob has earned 374 employee of the month awards.
If he was employed at 16 years of age (he has had a full childhood according to other episodes), that means he was 47 during the Spongebob Squarepants Movie.
He has worked at the Krusty Krab for 31 years and 2 months.
He started working in a November
EDIT: In Employee Of The Month, the 25th episode, Spongebob reveals that he has 26 consecutive employee of the month awards. The episodes were, on average, one month apart each in the Spongebob universe.
If Spongebob earned 374 employee of the month awards by the movie, that means that the first 117 episodes were 3 months apart on average.