How long are you hospitalized for having severe dehydration?

It depends. It depends on the cause of dehydration. Some will cause ongoing fluid loss, so correction of dehyration will take longer. Others are more simple and can be corrected over 24-48 hours.
Hard to say. If you are simply low on fluids, an overnight stay might do it. However, when this is triggered by a true medical problem, like diabetes, diabetes insipidus, secretory diarrhea, or other problem it could be days or weeks. When you provide little or no background information for your problem, it limits the specificity of any answer.