Hello,
I believe youve got a little mixed up. 6 years is the time period in which you can take action from a breach of contract/condition under the Sale of Goods Act.
The actual adverage lifespan of a product is determined from many factors: Product claims, the brand image, materials used, life expectancy of similar products under similar conditions of use, any marketing information used to give an impression of quality (including verbal information) and lastly the price of the goods. Though price cannot be a sole determining factor, it can be used to make a slightly better judgement, as goods to the value of £10 shouldnt be expected to last the same length of time as goods at the value of £1000.
The lifespan you determine is a subjective one, and therefore is up for debate between yourself and the other party, you must not be biased (being your product) and must think of the product from an outsiders point of view.
Personally I would estimate the lifespan of a Nexus 7 (dependant on model) to be somewhere between 2 years and 3 years under normal use.
Hope this helps