How far to push the ISO on the D800

I shoot primarily in the studio.

doing a shoot tomorrow in an office setting. will use the D800 witrh the 24-70, a SB910, with a Gravlon diffuser.

probably not great light. I'm thinking of shooting shutter priority at 1/125. to get a decent apperature, will have to piush the ISO.

So - what is your experience? How high can I safely go? I've had good luck reducing some noise in Lightroom, but don't want to push my luck.

thanks so much in advance for any guidance.