Question about double-entry visa to Thailand

Hi! 

I want to travel to Thailand on a double-entry visa and travel around the neighbouring countries after my first stay in Thailand. My question is: How long can I stay out of Thailand and travel and then re-enter for the 2nd period?

Does the double-entry visa expire after a while, or can I even travel for say 3 months through other countries and then, when I come back to Thailand the 2nd entry of my visa is activated?

Thanks!

Franziska