I made 150m in around 10 days once from attic ABing. It depends on what your goals are I guess. I'd say you can never have enough NPs, but if you have too many you'll probably get bored since you have nothing to work for.
When I have like 50mil in bank I start to think "what is the point" lol. I realize that the number can look good when it gets large but even with 50mil you can buy quite a bit of stuff.
I've never had more than 10mil on one account at one time, but that's just coz I'm paranoid (:
The way I see it is that 10mil = $20, and I very rarely carry more than $20 in cash in my wallet, in case it's lost/stolen.