I had a seroma in my axilla, what I could feel was bigger than a hen's egg. I'd had a small seroma, but it came up like that almost overnight a few months post op. It was uncomfortable, and I had it drained once. It reaccumulated so quickly that it wasn't worth bothering again.
I also developed one at the tumour site that I had drained a couple of times.
3 years post surgery, both are still there. I can't feel the one in my axilla, and the one in my breast.... well, the advice is that if I had it drained, my breast shape would be significantly compromised. My decision? Don't bother draining it! I'll keep the breast shape, thanks.
Other than an infection during radiotherapy, it's never caused any problems