only strong spirits can manifest. weaker ones can be seen as orbs or not at all. you also need to be clairvoyant, it'll help you see them. some people can only hear spirits, or sense spirits, if they were to fully manifest before them, the person would just be able to sense then stronger or hear them clearer. perhaps they could see something, but they wouldn't actually see the spirit, so you need to work on your ability to do so.
as i said, it depends on the spirit, it could take a few minutes or be instant. around this time the veil between our worlds is thin, so it's easier for them to manifest and us to see them. it does take a lot of energy, so don't expect a spirit to forever stay like that, they might go to a slightly translucent state, or transparent. i've only even seen two spirits manifest fully in front of me, one was very powerful, the other spent her time in one place, not doing much, so she conserved her energy, but she spent most of her time transparent, or not even visible, though i and a few others could sense her.
it depends once again on the spirit if it wishes you harm or not. i've never been attacked by any spirit, so i couldn't tell you. you should always protect yourself, a circle or a charm would work. why do you ask? is there a spirit you're wanting to contact? if a spirit was strong enough, and they wished you harm, they don't have to manifest to do it. an unseen force could push you down the stairs, knock something off a shelf landing on you, or out right attack you leaving cuts and bruises. this isn't common, most spirits just come and go, they live their life, don't want to hurt anyone, just want some company.