I agree with all saying the comma isn't
necessary. However, it is correct:
Comma Rule Number 2: A comma is necessary to separate a long introductory element before a main clause. Rule 2 holds true for both simple and complex constructions:
Even though ignorant of our culture, we must always be kind to strangers. (Simple)
Since Constance is new to our company, all of us should strive to help her. (Complex)
It’s
always correct to set off any introductory element with a comma, but a more modern lean toward fewer commas in novels has made the practice optional for shorter elements. Either is acceptable:
Later, you can join us for dessert.
Later you can join us for dessert.
I've also heard Pellshek's guideline; except, it was five
syllables before the length of the introductory element warranted a comma.
Good advice all around.