how much longer is a bilateral versus unilateral?

I wrote my surgeon a note yesterday saying I would really like to have a bilateral mastectomy. She had said she thought a uni would be so much easier on me and safer, but was open to doing both. She also suggested doing one now and one later. I am not having any reconstruction done and my breasts are fairly small (I am an old thing, they have "gone south" so so speak). Would the double be one or two hours more than the single? In general, what is the duration of these surgeries? Thanks.