Just once, according to the Starr Report. The other times she complained that Bubba wouldn't let her finish. As she left, she saw Clinton masturbating into the sink. GWB wouldn't enter the Oval Office without a suit and tie, even if it meant going upstairs and changing clothes. Clinton jerked off in it.
Hillary must have suspected that Monica was collecting DNA, and told him to keep it away from her. So he did, while continuing with the sex anyway.
The very definition of "sordid."