How much life insurance do I need? How long?

Hi,

I am 31, married, in decent health (don't smoke). No kids, but planning for one next year. How much life insurance do I need? Thinking about a term policy for $500,000, but not sure if I need it for 10, 20 or 30 years? Any advice much appreciated.

Annual income is the $100,000 range.

Thanks.