Guide for those that don't know: for every randomly selected typeable character you add to you password, it will multiply the number of combinations by around 60. So if your password is 3 characters, that means 60 * 60 * 60 = 60^3 = 216000.

Computer systems exist that can crack passwords by testing each combination one by one (a.k.a. brute force) and their capacity doubles roughly every 2 years. Currently the best of them are assumed to be able to crack passwords of 12 randomly selected digits within "reasonable time". These are farms of hundreds of computers each testing combinations at millions per second for days, months and years.

So if you select a password with 20 randomly selected characters, you will have a nice margin of 60^8= 167961600000000 longer time to crack than what is reasonable by the best technology of today.

You can stay at this edge by adding another randomly selected character every 60/2=30 years.