How do I know if my aeropress is too loose? I bought my wife an aeropress for Christmas and I used it a few times before understanding exactly what the instructions were saying - (I tried to make a latte with hot milk and water in the hopper) it was very hard to press down at that point - I've never tried doing that again, but I think since then it's been a little too easy to press down the plunger. 

Should it actually take 10-60 seconds to press down all the water given the pressure from above? It seems like almost all the water (up to #2) is through the filter by the time I get half way down the thing and the last half of pressing gets easier, not harder as you might expect with more pressure.
How easy it is to push the aeropress plunger depends mostly on how coarse or fine your coffee is ground. It sounds like your grind might be too coarse, a finer grind will make it "less loose". A medium-fine or filter coffee grind will usually work best in this situation.

As another answer explained, don't put milk into the aeropress, it won't filter properly. You can add milk to the cup afterwards if you like.

About 20 seconds to push the coffee through the aeropress with light to moderate pressure (just leaning on the plunger) would be a good time. Your description of the water passing easily at first and the plunger being "too loose" sound like your coffee grind is too coarse.

Here is an excellent video showing how to use an aeropress brewer by expert baristo, Tim Wendelboe.