How fast would a rash subside when using: antibiotics, cortisone, antihistamines and topical creams at the same time? any chance it would do nothing?

Varies: A rash can only be treated if there is some basic diagnosis. The treatment of such is based on a dx or most likely cause. Different rashes clear in different ways depending on the cause. If wrong cream or ointment for wrong dx nothing may work or may have a reaction. see your dermatologist.

Rash: It all depends on the cause of the rash. If it is an allergic rash for something topical like soap for instance then topical cortisone cream would be a good option for itching and redness. Antihistamines would help redness and swelling for an allergy rash. Antibiotic ointment help for topical bacterial infections. If you are having shingles (herpes zoster) none of these will help.

Depends on the cause: Antibiotics are used for treating bacterial infections; cortisone cream is used for contact allergy and atopic dermatitis; antihistamines are helpful for allergy-induced skin reactions such as hives; topical creams are distinctive based on the cause of a skin rash. Doctors do not prescribe all these different items at one time. The duration will depend on the diagnosis, basically.