2 x 20mg/week is typical. I seem to recall NICE says 1 x 20mg/week, but that's not enough for many men (and you may need more if you don't start taking it until symptoms appear).
I was prescribed 20mg/day, but told to start at 2 x 20mg/week and adjust dose to symptoms. There's quite a lag between changing dose and effect. I did try going up to 20mg/day at one point, but my liver didn't like it, so I backed off to a lower dose. I wouldn't have known this if my GP hadn't thought to do a liver function test (livers tend not to give symptoms until it's really serious and they may be beyond recovery).
When breast growth/pain happened again after hormone therapy finished when my Testosterone returned, I snapped the tablets in half (so, 10mg), and took one each time the breast pain reappeared. Looking at my log afterwards, that was every day for a week, then every 2 days for a few weeks, then every 3 days. I continued with 10mg every 3 days for another 6 weeks after the pain had gone which shrunk back the breast gland growth, and I stopped taking it a couple of weeks ago, and no symptoms have returned yet.
I mentioned this to my oncologist and he hadn't heard of gynecomastia after testosterone returned, but was fine with me having gone back on to the Tamoxifen and wrote and told my GP that I'd found this did happen in other men too through support groups.
Note that Tamoxifen has no effect on breast fat growth, only breast gland growth.