From the FAQs here:

http://faq.gardenweb.com/discussions/2766566/how-do-i-determine-when-to-begin-sowing-my-plant-seeds

But for rare or uncommon seeds then one guess is as good as another since there are so many different variables involved. For example one set of environmental conditions, container size, ambient air temperature, and potting mix used will give vary different results than another.

One trick is to stagger the plantings by approx. 2 week intervals in your specific growing conditions. That way you are assured of having some transplants ready to at the proper time.

Dave

You look up the planting calendar for your state and city.

And you can usually extrapolate from related plants that have the same origin or culture requirements.

Scheepers Kitchen Garden Seeds has a useful one pager Scheepers' When to Start Your Seeds. Annuals generally grow quickly - pretty much if the plant originated in a warm climate - don't winter sow as it doesn't need cold - you can use the jug or whatever method, just wait until danger of frost is past (some seeds rot if cold and wet). If starting indoors using a heat mat and lights, use Scheepers' guidelines. If it's from a cold climate - winter sow. For uncommon seed, check the plant family (to find a match) or ask here. 8 weeks is a rule of thumb for most, but I like my peppers and eggplants a bit bigger - so I start sooner.