1. In general strains that are top cropped commercially are poor bottom croppers. WLP300 is a notoriously bad bottom cropper.
2. For me about 1.070 or 60 IBUs and I won't reuse. Most of my beers are under this so its not a problem. Russian River is basically making Pliny and Blind Pig at their production facility so they economically have to repitch from bigger beers, so it depends on what you brew.
3. Assuming you start with liquid yeast (anecdotally dry yeast can't be reused as much), you should get ten generations on average if your practices are good. This varies a lot by strain. In practice, you want to observe the fermentation characteristics and stop using it when it starts behaving differently.