It takes at least 7 days for Zyban to build up in your body. It is suggested that you set a quit date during the second week of medication use. While you are waiting for the medication to take effect, you should work on a quit-plan that requires action on your part, such as building a support network, tapering down the number of cigs, etc. If you are extremely motivated to quit and do not want to wait, you can quit after using Zyban for a few days - although the medication may be less effective.
Source: Zyban (buproprion hydrochloride) Sustained Release Tablets Prescribing Information. GlaxoSmithKline, Research Triangle Park, NC. 3/2003. Accessed 11-08-2003.