Does an extended hold notice has need to be mailed out each time a deposit is made or only when we begin the six month extended hold period?

An extended hold notice must be given to the customer at the time of deposit, or, if circumstances become known after time of deposit, within one business day of becoming aware and placing the hold.

But I'm not sure what you're asking in the 2nd part of your question, because none of the holds are for "6 month" periods. What is your exception?

Only once...

12 CFR § 229.13(g)(2) One-time exception notice. In lieu of providing notice pursuant to paragraph (g)(1) of this section, a depositary bank that extends the time when the funds deposited in a nonconsumer account will be available for withdrawal based on an exception contained in paragraph (b) or (c) of this section may provide a single notice to the customer that includes the following information —

(i) The reason(s) the exception may be invoked; and

(ii) The time period within which deposits subject to the exception generally will be available for withdrawal.

This one-time notice shall be provided only if each type of exception cited in the notice will be invoked for most check deposits in the account to which the exception could apply. This notice shall be provided at or prior to the time notice must be provided under paragraph (g)(1)(ii) of this section.

Extended holds cannot be placed for six months

I don't think that's what is being referred to. If I read it right, the question is about an exception hold for repeat overdrafters, or --for business accounts only -- for large check deposits or redeposited checks. The regulation permits a single notice valid forever for large check deposits or redeposited checks (again, for business accounts only), and a single notice valid for a period of up to six months for repeat overdrafters.