My experience is that this varies much from concert to concert, and it may even change from country to country, or venue to venue. My best advice is to be there at least before the gates actually open.
When I went to the Antwerp concert in 2005, I was there around 18:00. There were already a bunch of people waiting before the gates, I guess at most 50 to 100 people.
When the gates finally opened around 18:45 the crowd had swelled to around 200 people. Then we showed our tickets and ran to the concert hall. We managed to get a good spot along the catwalk, only one row in front of us.
Make sure you don't have any bags or heavy coats that you need to leave at a counter, this can cost valuable time, travel as light as you can. Security might want to check your bag, this also costs time.
Also, it can get quite hot in the front crowd, so wear light clothing, or wear something like a sweater you can take off. Bring a bottle of water, because it may be hard to get drinks when you are in the front, and you don't want to leave your precious spot. Try not to drink too much before the concert, if you need to pee during the concert, it may be very hard to reclaim your spot!
Flick of the wrist