Well I prefer shorter elections. Later in the game I switch to monarchy usually.
Short elections (2 years) basically translates into 2 MP/month if you always re-elect.
Also it works great with the government reform that gives +5% loyalty to all estates on re-elections.
Finally it is just the greatest in terms of upgrading your ruler ASAP.
As a republic your rulers are aged 30-60 or something. After reaching the age of 70 the yearly death chance is 33% so for convenience's sake I take 72 as the maximum age limit.
That means in the best case scenario your ruler is ruling for 42 years. It takes 5 election cycles to fully upgrade your ruler. In the base case that would be 20 years. With short elections it becomes 10.
So short elections give you a max of 32 years of 6/6/6 whereas long elections give you 22 years.
Now if your fresh 1/1/4 is 40 years old, then your max lifespan is only 12 years for the long elections. And that is not even accounting for premature deaths, this is just maximum lifespans.
Also if you are playing republics you either want your republican tradition to stay close to 100 OR keep it floating between 40-50. The reason is that between 40-50 there is no chance of getting a dictatorship but there is an even 'Republican Traditions Debated' that lets you trade 1 stab for 20 tradition.