I need some information.

My non B and work permit are both due for renewal at the same time in late July.

I have been told "The application for the renewal of the work permit must be done at least month before the current work permit [and visa] expires."

Is this correct?

The regulations require that the application for renewal (WP5) is filed before expiry of the current WP/Visa; minimum turn around time is 3 days, but it can be much longer if there are issues with the paperwork.

Note that both an Extension of Permission to Stay based on Working in the Kingdom of Thailand (Police Order 777/2551 - Case 2.1) and a re-application for a One Year Multiple Entry Non Immigrant Category 'B' Visa require a current Work Permit.

So an application one month before will do no harm, renewal will be from the current WP expiry date.

Rules seems to differ a bit about the time frame depending on the lbour office you deal with. But always extend a few weeks before it expires.