Skill Deterioration

Author's Note: I have devised some quick & dirty rules for skills deteriorating over time with disuse. A lot of the numbers below are ballpark figures, not really reality-checked. I welcome comments and suggestions for improvements.
Note that I came up with these rules to handle immortals in my online GURPS Highlander campaign (they tend to accumulate lots of skills), hence the table extending for periods of time unlikely to be a factor in most campaigns.

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 the last column. Only the skill level is relevant, not the amount of character points spent on it.

[Notes on the table: 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.]

Skill Deterioration Table

Lang.	M/A Skills	P/A Skills	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.