Hm, that's not really a question you can properly answer. With a quick google search, I found out that horses are usually pregnant for 11-12 months before giving birth, but considering that he's clearly not a horse, we don't know if it'll follow this. In the real world, it's not possible for something to reproduce with another species (without special scientist shenanigans), so we don't know exactly how the baby will form. Would it take 9 months, assuming Loki's little swimmies are Human based? Or would it take 11-12 months, since it's forming inside a horse? Would the fact that it's forming into a horse-like animal cause it take longer, since it's significantly larger? I'm not exactly interested enough in this to delve into fetal growing patterns and approximate it, but there's some food for thought.
Also Loki was the mother