That depends on what your baseline is. You can play around with numbers here: http://bikecalculator.com/
Keep in mind that your aerodynamics may differ from their assumptions, but it’ll give you a good ballpark.

There’s no general estimate because of the number of variables. You need to fix the baseline power and the distance being traveled. Best to plug some numbers into the bike calculator linked above and compare.

It’s $60/year, but BestBikeSplit is THE tool for elite TTers and triathletes. Several variables you can adjust to answer all of your time savings questions. As someone who likes hilly routes, I use it to determine my Normalized Power required for a route to be completed in a fixed time OR for a given NP, how long will the course take me to complete.