Whenever there can be a change for the better, and government won't allow it.

When the government no longer represents the interests of those they govern.

Revolutions are justified when people are not able to obtain what they should be able to get. For example, Americans revolted because of "taxation without representation." This right was unjustly kept from them, thus justifying the revolution.

Revolutions are rarely justified. I'd even go so far as to say that the American Revolution didn't have sufficient impetus.

Revolutions are rarely bloodless (over 100,000 died in the Revolutionary War) and there's no guarantee that the new government is any better than the previous government - we suffered through the Articles of Confederation for a decade. A revolution is often a risky and bloody gamble. Things have to be really bad for the 'risk to reward ratio' to even out.

Generally speaking a revolution could always be argued as justified if, as was the case at the founding of this nation, the People are seen as the sovereign power within a given nation. However, I believe revolution is justified after "a long train of abuses" has occurred. If a given government is operating outside of the powers that the People have delegated to it and is disregarding the original agreement between the governed and the government then the time has probably come for revolution. Also, regardless of the original agreement between the People and their government, if the Natural Rights of the People at large are being violated routinely then revolution is justified.

Revolutions need no justification, they are simply the reflection of the material conditions of the present society.

"Men make their own history, but they do not make it as they please; they do not make it under self-selected circumstances, but under circumstances existing already, given and transmitted from the past." - Marx from the 18th Brumaire of Louis Napoleon.

Revolution is the rebellion of one class against the rule of another. It is the call for the birth of a new society. So long as the material conditions of a particular society necessity and give rise to revolutionary movements they need no justification.