What works for me is to not attempt to time it at all. Simply place two fingers directly on a message. Leave them there. Wait a second, then start sliding down. The message list should immediately shift to the right, exposing selection "circles" at the left edge of each message. As you continue sliding downward, each circle you "pass" will become selected. You can continue holding as you reach the bottom of the screen to keep the selection going. If you overshoot the end of the area you want to delete, you can slide back up to unselect a few rows before releasing. This works in both Mail and Messages.