Assuming this is an overdraft program covered by Regulation E, I don't know of any limits as the rule says the following:
"(iv) Provides the consumer with confirmation of the consumer's consent in writing, or if the consumer agrees, electronically, which includes a statement informing the consumer of the right to revoke such consent."
Also, I would document each request as you need to prove they had opt'd in for each OD fee you charge.
Adam Witmer, CRCM
All statements are my opinion, not those of my employer, and should not be taken as legal advise.