There are different grades of urticaria - acute urticaria can last from hours to about 6 weeks an is usually caused by some form of allergy, for example pet hairs, latex, foods. Bee and wasp stings as well as some viruses can also cause acute urticaria.
Chronic urticaria is more difficult to diagnose because it seems to be brought on by physical reactions within the body - similar to an auto immune disorder. Also, urticaria is triggered by parasites, and some illnesses such as thyroid disease.
If the urticaria is chronic then it's best to have it treated by a medical practioner. However, if it's acute and the symptoms are mild, then there are number of herbal creams, lotions etc. that do seem to be effective with many people. If you search online you will get quite a comprehensive list of medical therapies you can buy. Hope this helps.