Unlike flight hours, an instrument rating does expire. In fact, it only has a very small period in which the rating is active because after 12 months it expires. This applies to the first issue of an instrument rating and also to revalidations. So you will have to get your instrument rating revalidated every 12 months.
To be able to obtain an instrument rating in the first place then you will have to achieve at least 50 hours (preferably more) of cross-country flight time. You will also have to do at least 10 of these hours in an airplane if you want to get an instrument rating. If you have done this, you can get an instrument rating which you can then renew every year to ensure your flight abilities are kept up to date and that you can pilot planes to the absolute best of your ability.
If you want to work as a commercial pilot and have the money behind you then it is always a good idea to get an instrument rating behind you. But ensure you have the finances to support this as you will have to renew the rating every 12 months in order to keep it active.