Skills that aren't used regularly will degrade over time. Physical and Mental skills both degrade with lack of practice, but Mental skills tend to degrade faster. The higher your skill level, the more practice you need to maintain your level of training or knowledge. The more difficult and esoteric the skill, the more quickly a person forgets details. Language skills deteriorate fastest of all -- "use it or lose it" is very much a fact of language learning.
The table below shows your skill level, compared to the base attribute, and the amount of time that can pass without practicing it before it will fall to the next level. The minimum amount of practice time required (on average) is in parentheses. (Study time counts as practice.) Only the skill level is relevant, not the amount of character points spent on it.
[P/A skills are used as a base; it's assumed that a P/A skill at DX level will degrade (to DX-1) after 10 years of disuse. Mental skills move up +3 on the chart, so a M/A skill at IQ level will degrade to IQ-1 after 3 years of disuse. Language skills are an additional +3 on the chart; a M/A language skill known at IQ will degrade to IQ-1 after 6 months of disuse.
For Hard skills, move up an additional +1 on the chart. Very Hard skills move up an additional +2. Easy skills move down -1. Native Language moves down -2.]
No skill can deteriorate below its default level (which means one's native langauge will not deteriorate below IQ).
|Lang.||M/A||P/A||Time to deteriorate
(minimum practice required)
|+4||+7||+10||1 day (1 hour/day)|
|+3||+6||+9||1 week (1 hour/day)|
|+2||+5||+8||1 month (2 hours/week)|
|+1||+4||+7||3 months (1 hour/week)|
|+0||+3||+6||6 months (1 hour/week)|
|-1||+2||+5||1 year (2 hours/month)|
|-2||+1||+4||2 years (2 hours/month)|
|-3||+0||+3||3 years (1 hour/month)|
|-4||-1||+2||5 years (1 hour/month)|
|-5||-2||+1||8 years (1 hour/month)|
|*||-3||+0||10 years (4 hours/year)|
|-4||-1||12 years (3 hours/year)|
|-5||-2||15 years (2 hours/year)|
|*||-3||20 years (1 hour/year)|
|-4||50 years (1 hour/year)|
|-5||100 years (1 hour/year)|
* -6 or default, whichever is more
Deterioration is cumulative; the clock doesn't "reset" each time the skill degrades! So if you were once an Olympic champion fencer with Fencing at DX+8, it will fall to DX+7 after 1 month without regular practice, DX+6 after 3 months, DX+5 after 6 months, etc. If you neglect the skill for 8 years, it will be down to DX level. If you somehow lived for another 100 years and never picked up a foil in that time, your Fencing skill would fall to DX-6.
It's easier to relearn skills that have degenerated with disuse than it is to learn a new skill. The GM might cut the training or character point cost of raising a skill back to previous levels in half or more.