Do miscarriages typically happen over a couple days or all at once? If it varies, do you have a percentage? Thanks

Varies. Typically, a woman will start spotting and cramping. This will be followed by passage of tissue along with blood clots after which pain and bleeding subside. This process can take place in an hour or two but may last up to 1-2 days. Sometimes, a woman will continue to bleed because she does not pass all of the pregnancy tissue in which she may need a d&c.
Depends. Most cases are latent in utero (missed abortions) for several days before cramping and bleeding start. One these symptoms occur the products of conception are expelled within a few hours/days.