WGT - How Long
This is an interesting one, technically the festival starts Thursday afternoon and ends after Monday night, however you need to be in Leipzig before and after so you can get away with 5 nights, if you have then energy then this is the minimum you need (you cant buy day tickets to the festival, so if you wanted to just go for the Weekend you will still have to pay full price)
Personally I have been going to WGT since 2002 and have previously always travelled out on the Thursday and back on the Tuesday, however last year I decided to treat myself and go Wednesday to Wednesday and it made a real difference to my experience. First, if you are a social media user then travelling out to Leipzig on the Thursday means you are going to see all them posts from people already in Leipzig (whilst you are still working and wishing you were there already) and secondly WGT is not easy, its brutal, long days on your feet, travel from venue to venue, dancing, drinking, having the time of your life and being outside lots, just the travel too and from Leipzig is hard work (esp. if you are north of 50)...so my advice is give yourself the extra days if you can afford them, a day to recover from the travel to Leipzig will help, a proper sit down meal before hectic days of travel and festival food, and that day after the festival is essential, time to put your feet up and chillout before making that trip home.
If you are travelling long distance then you will probably have plans to visit other parts of Europe, I would highly recommend spending time in Berlin if you can, there is so much in the city to see and do, so much history and museum's to visit.