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.
The following details of the assassination timescale is taken from a post by Fawr on the official Paradox game forum.
So looking at the events there are a number of ways a plot target can die.
- Poison - 400 years*
- Falling - 300 years*
- Archers - 700 years
- Explosion - 700 years
- Break into Prison and kill - 100 years (prisoners only)
- Snake - 700 years*
- Balcony Plummet - 700 years*
- Disguised highwaymen - 700 years
- Carriage Accident - 700 years*
The ones with *'s only happen for plotters in the same province as the target. You can reduce the years by having extra plotters (so 10 plotters will reduce the average time to 1/10th). 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). So 100% generally reduces the plot time of another plotter from 700 years to under 7 years.
Then there is a chance that the target survives, dies, gets injured, etc.
There are also quite a few extra ways of killing targets who are young. The ones above are the ones which apply to adults.
Looking at this you need lots of plotters, not just good plotters. Plotters who are in the same location as the target are more than twice as likely to succeed. With only one other high intrigue plotter in your court helping you it is likely to take forever. 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.
If the plot does go ahead then paranoid and diligent targets are less likely to die, but trusting ones are probably out of luck.
The following strategy advice is taken from a post by Valentinan at the Paradox official forums.
Plotting and assassination are talents to be developed, like everything else in the game. To begin with, your character needs at least 12-15 intrigue to be successful most of the time. In addition, spymaster's of 25 or greater are extremely efficient, those of 20-24 usually work well, and those of 15-19 are somewhat unreliable. Crown intrigue above (40) is the bomb, 35-39 is very good, and 30-34 is perhaps the minimum necessary for success.
Sometimes assassination is better than plotting. First consider assassination attempts with less than a 5% chance of discovery as they almost always work. Those with 10-12% chance are risky but doable, and anything above that should be plotted instead.
Prestige. Prestige of 1K is minimum and 2K or more is preferred. When characters are asked to join a plot they will consider the prestige of the plotter. That newbie king with 100-200 is not going to get the job done, so forget about it.
Court plots are the easiest to manifest. Court is a marriage factory, characters come and go, but while at court they are under your thumb. Consider training all your daughters as potential spymasters. If you don't have daughters, then invite some high intrigue females (or males) to court. Don't start any court plots without having 5-6 potential co-conspirators with intrigue above 15 (20+ is way better).
Gold is the grease that makes plots work. Only consider plots starting potential of 30% or more. Start by sorting list of co-conspirators by intrigue, and only consider co-conspirators with intrigue of 10 or more (7-8 at the very least). Only activate plots of >100 plot power, and if there's not enough characters to bring plot power to 100 then start spreading the gold. If you have to worry about how much gold it may cost, then forgetaboutit. If you can't bring the plot up to 100, then cancel the plot.
The target's intrigue and traits are other factors to consider. Co-conspirators are sometimes hard to find when the target is well respected. Generally you'll get a sense of this when plot power starts at something less than 30. Tread carefully here, and consider assassination first if this is the case.
Plots within the realm and plots outside the realm are tricky. Always position your spymaster in the target's county when considering assassination (duh!). If the odds are against assassination, then proceed to plotting. Before starting a plot consider the target's court. If the court is small it may be hard to find co-conspirators. Also consider the court of the subject's liege (the duke) as potential co-conspirators. If it's one of your vassals, then consider your council as potential co-conspirators. Never ask drunks, imbeciles or lunatics to join a plot, and understand the more people involved the more chances it will be revealed. Time is also a factor. Pause the game while passing out invites. Then, as soon as plot power gets to >100, pause the game again to initiate the plot.
Practice, practice. Plots are a great tool to use for arranging marriages, deposing heretics, acquiring baronies, etc. Practice the art on "old" women in your court, or otherwise excess characters no longer useful to you. Outside of court, practice on heirs to cities or baronies, or just random character in the realm (or outside the realm for more of a challenge). Spending gold in your court may come in useful later, but otherwise don't waste gold on characters you may never see again.
Also, dont' forget to check the "known plots" listing. Your character may wish to help a plot in progress, or just give gold to the plotter to aid the process.