generally, i'd say 500% is WAY too high. however, i have one campaign that i run that way - it's affiliate direct-to-merchant links that "competes" with my merchants own ppc. Most of the time, my ads hardly show at all because i'm bidding for roi and they've got a larger margin inhouse and different metrics / goals and they outbid me by a huge amount. But, they often run out of funds and shut off their ppc completely and then my $100 / day spend goes to $500 / day. The merchant actually likes me being there as a backup plan so they're traffic doesn't disappear when they go dry on funds. In this case, 500% over my normal full day spend makes sense, but it's an unusual case.
i'd say my average for established, stable (running more than a month or two) campaign is somewhere in the 120% to 200% (max daily budget / expected full day spend) when i want to make sure i run all day through peak volumes that may occur. the volatility of the campaign's daily spend is what drives me up towards 200% when it's necessary to do so to cover all peak days.
all this said, risk and reward and best determined by you and you alone. err on the side of caution and you'll smile more when bad weather comes - and it will come.
