100 yds is the "standard" for backing. I'd recommend at least 50 yds just to be safe....but for bass you probably will not need it. An 8wt is, IMO, overkill for bass. You might wear yourself out casting that all day. I use a 6wt. It'll cast everything but perhaps the biggest/heaviest flies. One trick some bass guys do is to overline a 6wt with either a 7wt or 8wt bass line. Usually one backcast is all you need to fire out a good-sized fly.

That 8wt would be very handy if you find time to get over to New Mexico to chase some Tiger Musky.

Good luck!!