Tough tough tough question. When it's acceptable to rebel against a government is going to be different for different people. Some people would say never. Some people might say every 25 years for whatever reason peaks their interest. Personally, I think it would be acceptable to rebel against a government when that government is intentionally doing harm to its citizens by making it next to impossible to acquire basic necessities of life -- food and housing. At that point the health and wellness of a population is in decline. Lives are at risk because the government is hoarding or pricing things too high etc. I'm quite certain that type of government could exist in any format -- dictatorship, democracy, communism, monarchy, etc. I don't believe any one type of government should be overthrown based on its type, but if basic quality of life services are being denied from citizens by the government, then I would consider it acceptable to rebel.
When your basic human rights and freedoms are removed, as in the South African Apartheid system.