Duration of rank

When you have for example shodan and you quit aikido, and after lets say 10 years you start again will you still be rank of shodan? Because after that time you will probably forget a lot of things.

That depends on the policy of the style and the individual dojo. I've seen some ranks respected, others forced to start over, and others who would have had their ranks respected request to start over. Talk to the teacher in question.

I have never heard of anyone being stripped of rank due to absence of training. It´s not like all shodans within an organisation are at the same skill level anyway.

Never heard of that - the being stripped part.

What I have heard about is Yudansha returning after a long absence and using the curriculum to drive their own training to bring themselves up to scratch.

I doubt this situation occurs so frequently that the other members of the dojo would be confused by a strip of cloth. This depends on the teacher I guess but budo training is in the end self training.

I do remember being very impressed years ago where a mega-dan purposely stopped wearing his black belt to foster what he called the beginners mind.

Rank is for life as (under the Aikikai system) Shodan registration includes lifetime Hombu membership.

I've had the situation of "returning" Dan grades crop up several times - not usually a problem as everyone respects the achievement but not necessarily the current skill level. Also if the wish to be promoted they must wait either the full time as mandated by Hombu regulations or until I think they are ready. Has led to a couple of difficult decisions (on their part)

I heard a prominent student of a famous shihan closed her dojo after an illness caused her to lose her technical abilities. That was her choice. Loss of skills didn't cost her rank, but it did cost the ability to run a dojo in her eyes. Students are even promoted post-humously, so even being dead does not cost a person rank or their ability to be promoted.

I have heard of requests to remove a person's rank because of criminal activity. This can get you thrown out of a particular dojo or association but rank (and really the skills acquired) stay.

That said, if I didn't train for years, everyone I trained with initially will have either quit or continued on - either stopped or continued to improve. If someone was a lower rank when I left, they are a higher rank now and deserve to be treated by me as such. Testing standards and preferred variations will have changed, and I would have possibly forgotten the ones I ever knew anyway. I should expect to be treated as a low skilled guest by the white belts. There should be no chance that I could teach, no pearls of wisdom being offered by me to any training partner. I might not be physically fit enough to take the advanced classes. And that's just if I returned to my own association - if I took years off to start again in another association, all the ghostly memories of techniques might be completely different than what the teacher wants.

The rank is there, but demanding the respect and perks of rank would be a poor choice.