Most Arab and Muslim countries have their leaders for decades in power. Is this the best, or even acceptable rule?

On the contrary, rulers of developed countries serve, at the most, two terms of 4 to 6 years. Who is right?

In Yemen, and many other Arab and Muslim countries call themselves democracies, a president stays in office until his people – and in some case international powers – think that no one else is able to substitute him.

Internally, citizens get used to see no other prominent figure in the political scene, as the one who is already in the power determinedly clear the atmosphere from any shining star (either by buying influential characters off or by liquidating them). Even in elections, if any, people find themselves tend to elect the "president", who has stayed so long in office that they cannot imagine other person in his place.

Externally, a ruler seeks from the very beginning of his service to satisfy powerful states and fully cooperate with them so that they find him the only asset suitable for helping their policies dominate.

For both cases, the Yemeni example was the best; where many leading persons were co-opted, threatened or assassinated, while international players remained for fairly long time looking for an alternative (who is willing and able to cooperate in counterterrorism).
Now, even with the high turnout in the early presidential elections to have Hadi in place of Saleh and the fact that the West, of course, no longer with the latter, there still many Yemenis loyal to Saleh. It is true that some try to maintain their own interests, but many of them Saleh was so deeply entrenched in their mentalities that they cannot see Yemen without him, as they say.
All this proves that the longer a ruler in office, the less opportunity he could be changed in reality or in minds of the public.
Hence, Arabs and Muslims should learn from others' lessons. This does not mean that the Islamic world lacks excellent models for good democracy, such as Turkey, but many and many other more brilliant examples are not Muslims.
They learnt that a ruler must not stay in power for more than two terms, and authorities should not be given to one person or body.
Hopefully, some Arab countries have eventually find the right way.