When a company changes name how quickly do we need to change labels?

We'll be changing our company name soon. Combining with another company. How long do we have to change the name on our labels?

There are lots of producers that change corporate ownership, the brand name on the label stays the same because it has customer recognition value.  You are "doing business as" or using the former official company name as a "brand".  Even if you did want to change the name on the label, it is highly likely the new company retains ownership, trademark, copyright, legal liability etc. over the old name.

The only part that is likely to legally matter is the contact information, which may not even include the company name.  It is often just an address or consumer contact department.

I'm not 100% clued in on US legislation, but in the EU, you're required to include the address of the production site on the label.

Therefore, I would assume you need to change labels as soon as the name change is complete from a legal point of view. 

Have you asked the auditing bodies to update your certificates as well?