To comma, or not to comma.

As a matter of personal preference, I always use a comma following an introductory adverbial phrase -- "In a few hours, her father would be home."

However, here is what the Chicago Manual of Style has to say: An introductory adverbial phrase is often set off by a comma but need not be unless misreading is likely. Shorter adverbial phrases are less likely to merita comma than longer ones.

The MLA Style Manual just says: Use a comma after a long introductory phrase.  However, MLA does not attempt to define what is long.

Because the style manual guidance is a bit ambiguous, I tend to err on the side of caution, but it really is your call as the author/editor.