How long am I contagious with a cold/uri? Trying not to get co-workers sick and deciding when to return to work.

Judgement call. It really is a judgement call. As long as you don't have a fever and can control your cough and runny nose, you should pose little risk to your co-workers. You should wash your hands frequently and use hand sanitizer when washing is inconvenient. And politely refuse shake hands until you are better.