This is straight from the IRS:
"When a Form 8857, Request for Innocent Spouse Relief, is filed with the IRS, it may take up to 6 months before a determination is made. During the processing time, the Service is requesting your tax information and contacting the non-requesting spouse. By law, the IRS must contact your spouse or former spouse. There are no exceptions, even for victims of spousal abuse or domestic violence."
In my practice, it has been between 4 and 5 months for the last 2 years:(