Please refer to the answer of the following question:
Would the Zakat be applicable to both the husband and wife's gold?
When one lunar calendar year is complete during the time you had the jewellery, you need to pay for the Zakat. For the example that you have mentioned, if you had bought the Jewelry 8 month ago, then the Zakat will not be obligatory except after 4 month. After 4 month you have to give out the amount required. Except if you are using this Jewelry for wearing. In that case there is no Zakat at all.
There are 2 methods that the 4 major scholars kind of agreed upon:
First, you set a date in the lunar calendar, like the 1st day of Ramadan or Ramadan period for example and check your money at that time. If you are above the limit "nisab". And you have been above the limit the year before as well around the same time, then you pay the Zakat on the money that you have.
The second method, is to wait until your money reach the nisab and you start calculating one year from that date. Say it reached that limit in "Shawal or ZulHijjah" then you wait till next ZulHijjah and pay your dues.
I personally prefer the first one for easiness. You could pay the Zakat even before the 1 year is complete if you wish.