Tamoxifen takes 2 months to build up to the working concentration in the blood, and 3 months to reach the final stable level, because of its long biphase half-life. (I posted a long message on tamoxifen dosing here: https://community.prostatecanceruk.org/posts/t17616-Breast-growth-with-bicalutamide-and-tamoxifen#post215572)
It can reverse breast growth, but that's limited. The most recent breast growth is reversed quickly, but the longer it's been there, the less effective it is at reversing it, and it can't reverse growth over a year old. Also, it can't reverse the growth of fibrous tissue which forms to support the breast tissue. Breasts also contain fat, and I've seen nothing in the research articles I've read which states if it can remove breast fat, or even prevent it forming in the future, but it is effective on breast gland tissue growth (and pain) for around 70% of men with gynecomastia, and completely reversed the initial growth of breast buds in me.
It does have side effects. It increases risk of DVT (and usually won't be given to people at increased risk of DVT, although I'm not aware anyone checked that in my case - certainly they didn't ask me). Some peoples' livers are not great at handling it. I would suggest you get a full blood count and liver function test done when you've been on it for 4-5 months, to check your platelet count, and alanine aminotranferase (ALT) levels. It will push ALT high, possibly outside range, but a very high level combined with low platelet count would suggest that it may give you non-alcoholic fatty liver disease (NAFLD). Around half the women who take it for 5 years after breast cancer will get NAFLD, and a small number of these can become serious liver problems. (This may be more of a risk if you also drink alcohol - not sure.) I am trying to work out the minimum dose I can get away with which still works.