Dental flossing, essentially is done to maintain periodontal health, in hard to reach areas. Areas which are not used to any "friction" so, they will react in a rather singular manner in comparison to our exposed gums, per say. Thus, providing you presently have good oral health i.e. you don't have gingivitis which causes bleeding of the gums amidst brushing.
Initially, flossing will cause inflammation and minor bleeding however, if the bleeding is rather severe it could indicate a periodontal disease and require medical intervention. Gums, if they are "virgins" to interdental flossing scene, will be tender, and this is common amongst nearly all whom who haven't flossed. So, despite your concern, they will become more firm once the plaque has been plausibly removed. As your gums become less tender, they will hence become less sensitive and no longer bleed as you floss.
Albeit, if the bleeding still prevails after a period of a week, it would be advised to either, refresh your knowledge on how to floss, or pay a visit to your dentist. The latter could conclude that, you are incorrectly flossing e.g. too much pressure and rigor.
It's difficult to judge when you'll become apparent of the benefits since, the greatest improvements will initially happen beyond your eye's magnification. This study may cure your curiosity, though.