Hi there. Tickets for 2019 went on sale on the Sunday and were available for around 3-4 days before they sold out, so you do have some leeway. We got ours for this year just before they sold out on the Wednesday evening after the festival.
So roughly... 2pm WST time on that Sunday to get em... (2 years ago.. I got mine I think around 5pm.. got the xmas tickets.... Last year.. stayed up late in a Hamburg Hotel and got em midnight local time)
well wacken has been sold out a lot faster in the past, so don't count on it. If you really want to be sure you end up with a ticket just buy them at midnight.