How long will it last in the refrigerator after it's open

Are you SERIOUS? Until you finish it.

HOw long will last until it goes bad

Did you see an expiration date? Most brands have it stamped somewhere on the lid but you might need a bionic eye in order to find it. Jelly will usually be ok for several weeks after it's opened if kept in the fridge. If you're not sure how long it's been there then throw it out especially if you can't find any date. It's also possible that the date may be in some type of code which is put there to confuse the hell out of the consumer. If the jelly is real watery then this is another sign that it's time to buy a new jar.