Usually, the seat block only kicks in a couple of days before the flight (used to be at the beginning of check in, but for example now the block is there already on my DUS flight on Thursday). If somebody reserves a seat next to you before that, there's no block. I always check the seat map a couple of days before the flight and change, if there's somebody next to me. Would say recently I've had about 90% success rate, much better than before.