Unless you use time compression (which most people consider cheating, myself I use it only if I am testing the range/endurance of an aircraft) you take about the same time it would take in the real world (FS is a few seconds per hour off from realtime, so it's not exactly the same).
As to aircraft assignments, some VA do and some don't.
Most that don't will at least give a recommendation (like 737 for LHR-FRA and 747 for LHR-JFK) but leave the actual choice to you.
Best is to visit some VA sites and read up. Ask for information from them if you want clarification on something.
We have an intro online about how TradeWind Caribbean operates: http://ourworld.compuserve.com/homepages/tradewind/visit1.htm