Ive done both plastic and metal in simple green concentrate with not damage to the minis. Usually 24 hours is sufficient, but I have actually left plastic minis in the cup for up to a month with no damage, though I wouldn't recommend taking the risk.
It seems that once you get a couple days soaking it doesn't make any difference in how much paint it takes off. Most of it will wash right off, but a toothbrush and a small pick could be handy for getting into the deep creases.