How long until you start seeing actual weight gain?  I know in the beginning I'll probably bloat and retain some water, but when should I see my actual weight go up?  A week?  Two?

Depends on how much you're eating and how damaged your body is to begin with. If you are eating at about maintenance you aren't going to see anything for a long time. If you have a lot of weight to gain, your body is going to gain around your vital organs first and you may not see any changes in your body or on the scale.