I think Sophea is spot on.
If in your Doctor's opinion you are unfit for work, they can reasonably sign you off sick for as long as they like. If it's going to be for an extended time they will usually want to see you at regular intervals, maybe once a week, but I have been signed off several times for as long as a month between visits, because I "reasonably" needed to be off work that long.
Hope that helps.