Personally, I think that Saruman never had any intention of holding faith with Sauron, he just used his purported "alliance" to aid him in obtaining the One Ring for himself. It was basically a stall tactic--if Sauron thought that Saruman was faithfully handling the affairs in the region, Sauron would largely stay out of the White Wizard's affairs and leave his search unhindered. Sauron, of course, was no dummy, but even if Saruman found and claimed the One for himself, the Dark Lord believed (and probably rightfully so) that he could crush him regardless. In Sauron's mind, Gandalf was the much larger threat.
Which brings us back to Saruman's attack on Rohan. Saruman believed that the One Ring was definitely somewhere in the region. All signs point to this being true: Ugluk and company captured two hobbits near the Anduin (and Saruman knew a Hobbit held the Ring), the entire group of Uruk-Hai mysteriously disappeared, and Gandalf and Aragorn are found to be meddling in Rohan's affairs. Logic would dictate that Saruman's nemesis Gandalf had recovered the hobbits and the Ring. Seeing his plans being so beautifully spoiled, his only option was to launch an overwhelming attack against Gandalf in a hope to regain control of the Ring. That was his only purpose in striking out at Rohan.
So what next? Saruman had no idea that Gandalf and his companions did not have the One Ring. It wasn't even a contingency that he might've planned for. I'm not sure he would have believed it in the face of all evidence. If he had won at Helm's Deep, he probably would have spent the ensuing time trying to track down on how the One Ring eluded him once again. As for his alliance with Sauron, he would maintain faith as long as it would aid in his search for the Ring, and not a minute more.
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