I have had the bypass for almost 5 years now. My lab work was all normal before I had surgery and my bariatric doctor recommended the bypass to me to really get the most bang for my buck and I agreed since I was relatively young. I, however, needed both iron infusions and B-12 shots after surgery. Iron was pretty immediate after (I would say a month after) due to my symptoms. The B-12 shots I needed within the past year or two (it's been a minute). The shots were discontinued because my levels have been normal but my body has never been able to absorb iron any longer and maintain it. It was bad right after surgery and especially last year (but for some reason even though I had every single symptom of the deficiency, it still took me too long to resolve and my ferritin was at 4 and iron saturation was at 5), This time around dealing with these iron infusions , I said if I would have known it would have such a HUGE effect on my body (it was never really mentioned, and I never had problems like this before so I kind of turned a blind eye to it pretty much), I probably would have opted for the sleeve instead.
It takes a lot of advocating for yourself and support from your medical professionals and to be 100% compliant with everything to ensure you stay healthy along the process.
Remember everyone's body will react differently. I lost a lot of weight and REALLY fast after and have lost about 150 pounds total. Good luck!