Hot Air has a poll on the subject. It's an interesting question, I suppose.
If laws are just, then obedience to the law is an important part of one's duty. Where laws are unjust, disobedience of the law is often obedience to duty. Any violation of the law can be said to be a felony by whoever makes the law; and in a corrupt system, they are likely to make more felonious the most virtuous violations. There will still be thieves and robbers in the worst society, but political prisoners will pay a higher price the worse the corruption becomes.
So, when should felons be allowed to vote? There's no single answer, is there? It depends on whether what they did was a crime that would be universally recognized, or a crime against the politics of the corrupt. They may be morally unfit to ever vote. They may also be your best guides.