We do exactly what Swellman did. We tried 3 hours but found we were getting a low in there - so we backed off to 2 hours and it works pretty good.
That said, we use Humalog. I don't know if the different acting time for Apidra might mean differently. I would start with 2 hours and you might even end up backing up from there.
It definitely tames the spikes for us
Samantha - 17
Michael - 13
Emma - 8 (2/16/05 dx 3/07, dx Celiac 1/10) - Pumping with a Pink Animas Ping! as of 9/07
CGMSing as of 3/26/09 with Navigator, 9/14/12 with Dex 7, 12/5/12 with dex G4