How long?

bulldoggerel—I'm assuming this would be a lot like contacting an agent—an email nudge at the three-month mark would probably be okay. Always be polite:

Dear Publisher,

Back in January I sent my manuscript "Time Flies" to you upon a request for full. Have you had a chance to look at it yet?

As always, thank you for your time and consideration.

Yours Respectfully,
Anxious Writer

What's their posted time-frame for responding? Do they have a policy that they do or don't reply to all subs? Was it requested or simply submitted? All of those things affect the appropriate next step.

Other than Harlequin and its category lines (where, if your story really is appropriate for one of their lines, it's probably not appropriate for any other publisher), I'm not aware of any publisher who requires exclusivity, so I'd keep submitting, rather than contacting the publisher. I remember one publisher stating its response time was "four months to forever," and it really wasn't uncommon for people to hear a year or more later. I had at least one personalized rejection at the year-plus mark, while others were in the four to six month range. Personally, I would just keep submitting, and not check until AT LEAST six months were up, in case it was lost, perhaps more like nine months, but that's just me.

I'd nudge at the one year mark in this case. Publishers are notoriously much slower than agents. I have received rejections at the 18 month mark, even from e-mailed submissions on requested material. They were all very nice and apologetic because the editors knew they had taken forever.