I have not had a hysterectomy, but I have read that if your ovaries are taken out, you are thrown into immediate menopause with some symptoms. My mother had this done and she immediately went on HRT from age 45 to 74. She just went off of the HRT this year with no breast cancer problems, etc. The HRT was a lifesaver to her. Whereas if you have your ovaries that are still producing some estrogen, your symptoms could vary due to how much or little estrogen your ovaries are producing. You could just have mild symptoms. It is always best to keep your ovaries, unless there was cancer or something.
There are several ladies on this board who have had hysterectomies and they can give you first hand information on how it effected them. I think the length of menopause symptoms depends upon the individual.
If the doctor feels that it is in your best interest to have your ovaries removed, ask him about bioidentical hormones instead of HRT. I have heard wonderful things about them. I hope your surgery goes well and you are feeling better soon.