IRS Acceptance of Returns

On March 21 I e-filed  three Federal returns.  Two relatively straight forward returns were accepted by the IRS within two days.  The third return (about 100 pages) has to date still not received an acknowledgment from the IRS.  Because of its complexity, I can possibly understand the delay.  However, in the past ten years or so returns of equal complexity were still acknowledged by the IRS within two days at the most.  How long should I give the IRS before acting, if they do not acknowledge the return, and what is the procedure?  This return did receive the preliminary email notice that it had been sent (by TurboTax?).

 

Thanks for your thoughts!