I suppose the answer is it depends...on species, variety, environment and trait of interest. It'll also be difficult to separate out differences due to environmental factors and varietal factors.

I previously worked with grasses and before a variety can go to market in Europe it must go through a series of D.U.S. tests - to prove it distinct, uniform and stable.

Dozens of traits (e.g. flag leaf length) on individual grass plants of each variety replicated 10 times will be measured each year going back over 25 years.

Although they don't measure agronomic traits you describe, these sorts of datasets could provide you with information on the variation within a variety and also over years. 

This depends on the genetic nature of the variety, the reproductive system of the species (cross-pollinated or self-pollinated or vegetative propagation), and above all, the way of maintaining the variety during seed multiplication process.

if you have a good conservative selection system, whatever the genetic structure of the variety, you'll be able to keep its qualities as long as you want. Obviously, the conservative selection system also depend on the reproductive system and the genetic nature of the variety.

In short, I think that all depends on how you maintain your variety during seed multiuplication process. if the method used for seed multiplication preserves well the genetic identity of the variety, you can maintain the quality of your variety as long as possible

This depends upon the nature of crop and its propagation method. if crop is vegetatively propagated will preserve its genetic purity for a longer period, until mechanical mixture occurs or crop is infected with pathogens which may perpetuate for longer e.g. viral diseases. Farmers first time purchase the variety and further uses own multiplied planting material or seed without following the seed certification procedures may make the farmer think for replacement with fresh seed or planting material after sometime. If there is 100% seed replacement with the fresh seed and planting material raised through standard procedure of seed production passing through different phases of nucleus seed, breeder seed, foundation seed and certified seed, the variety will survive for long. It is always advisable to change the stock in phased manner with procurement of breeder seed from source after every 5 years.

If work is continuously done means maintain the cultivar for particular trait  then it never be deteriorate for longer time, in case of disease or pest attack untill they change their selves  

I think this depends on the genetic sources of this variety , is it the  hybrid , synthetic , open pollinated or self pollinate ...ext .  , the breeder methods used to improve this variety and the hertibility of each trait .

If nucleus stock is maintained properly with high genetic purity and full seed production chain is followed as per standard scientific procedure, the genetic purity could be maintained for a longer period without any genetic erosion.