To begin with, your character needs at least 12-15 intrigue to be successful most of the time. Plotting and assassination are talents to be developed, like everything else in the game. The following strategy advice is taken from a post by Valentinan at the Paradox official forums. If the plot does go ahead then paranoid and diligent targets are less likely to die, but trusting ones are probably out of luck. However half a dozen plotters, 100% plot power, some of the plotters in the target's location, and you are looking at a plot which would take on average less than a year to happen. With only one other high intrigue plotter in your court helping you it is likely to take forever. Plotters who are in the same location as the target are more than twice as likely to succeed. Looking at this you need lots of plotters, not just good plotters. The ones above are the ones which apply to adults. There are also quite a few extra ways of killing targets who are young. Then there is a chance that the target survives, dies, gets injured, etc. So 100% generally reduces the plot time of another plotter from 700 years to under 7 years.

Depending on the plot an extra 40-50% plot power will reduce the time somewhere near a half (for the plot leader) or a tenth (for other plotters).

The actual time needed to begin the plot event is base time / N, where N is the number of backers (so 10 plotters will reduce the average time to 1/10th).

Plots marked with ' * ' can only happen if the original plotter is in the same province as the target.

Break into Prison and kill - 100 years (prisoners only).

There are a number of ways a plot target can die: The following details of the assassination timescale is taken from a post by Fawr on the official Paradox game forum. Also note that the information provided here is relevant as of patch 1.06b, and may not be fully applicable if assassination plots are modified in subsequent patching.

Crusader kings 2 intrigue guide