There is no set percentage of bankroll, and it is very agressive which is why most use quarter-kelly or half-kelly so that you are being more conservative.
Use this calculator..
http://www.albionresearch.com/kelly/
So in your instance you'd enter $500 for bankroll.
Then for example, if you were interested in Newcastle pulling off the upset and winning tonight, you'd enter the odds offered in fractional form - 13/1 (+1300). Then conservatively estimate the percentage that you think Newcastle would win - i.e if you think they win 10% of the time when playing at Arsenal (1 out of 10 games, or 10 out of a 100, etc...) then you'd enter that, then click calculate bet and below it will give you a summary. With this example it will tell you that odds would be in your favour and optimal bet would be 3.08% of bankroll ($30). This is based on full-kelly though, as said above most use half-kelly or even quarter.
Obviously if your estimated percentage is way off then this doesnt work, but it at least highlights how ridiculous it is mathematically if you are throwing $250 on for example Newcastle to win when your bankroll is $500. If you did that it would imply that you think Newcastle win the game 30.5% of the time, which clearly is ridiculous.
And by no means am I an expert or anything, I've lost over 50% of my bankroll in space of a few hours before chasing losses where I completely disregarded the concept of bankroll management. That's the main reason I try to use kelly and that specific calculator as much as possible...it at least highlights the absurdity of certain bets that you have/want to make!