What is the time frame for notifying a customer of your findings after your investigation has finished? Is it three business days?

I believe you're correct.

Yes - 205.11(c)(2)(iv) Reports the results to the consumer within three business days after completing its investigation (including, if applicable, notice that a provisional credit has been made final).

Don't forget, if the claim is denied, the response needs to be in writing.

Randy's cite is correct but applies to a 45 day investigation. The same rule applies under §205.11(c)(1) for 10 day investigations.