we are taking the train from Venice to Milan. I am looking for a hotel near the train station so that we can check-in quickly and get to the last supper for an 615pm viewing. My second request is: How much time do we need between arrival of train, and getting to the last supper. We are trying to stay in Venice as late as we can.

We took the same train trip a month ago from Venice to Milan.
We stayed at the Hotel Una Century which is about a block and a half easy walk from the train station. I found the recommendation on Fodor's travel talk.
It was a modern high rise hotel with a restaurant and breakfast included. The front desk personnel were helpful and spoke english well.
It was clean and had all of the ammenities that american hotels have with the exception of an iron.
I'm sorry I don't know about how far to the Last Supper but the front desk can call a taxi for you. We took a taxi to the Piazza Del Duomo for about 6 euro from there.

We stayed at Hotel Berna close to metro and train station within a block and half.

We stayed at the Hotel Andreola Central and liked it very much. It's 300 meters from the train station - just a couple of blocks. Look for prices on www.venere.com, as the prices seem to be much better there. Also call the hotel and see if you can negotiate a more favorable rate.

We stayed at Mini Hotel Aosta across the street from the train station and it was like an American hotel. You have to check in for the last supper before your viewing time but I don't remember if it was 15 or 30 minutes. I don't have a map handy but I think it was only a couple of subway stops to the Last Supper from the central train station area.