Generally 3-4 outings of 3/4h if in flashing mode.

The Knog Plus is waterproof
I use it for switching, and the battery life depends on the flashing mode.
In intermittent mode and low power (enough to be seen), the battery lasts around 25 hours (I charge it every 2 weeks).

I've never tried this, because I use an asynchronous flashing variant that "hopefully" attracts more attention.
I can only answer the question about leak tightness to a limited extent, I ride a road bike and therefore don't ride much in splash water.... But it has never tarnished, i.e. water cannot have penetrated...