Full-time effort >> part-time effort. However, there are cases where part time is good enough.

It varies on how much (smart and well funded) competition there is, barriers to entry, your own personal ambitions. In general, larger and stupider the competition, the more likely you can do ok with part-time effort.

Joel's post on being a Ben and Jerry's vs. an Amazon can give some insight on this topic: http://www.joelonsoftware.com/articles/fog0000000056.html