New theme purchase help.

I have purchased a theme in envato market. I wish to use that theme for two different wordpress sites. Is it possible?

No, you can’t use the theme for multiple sites unless you buy the theme for every site. If you buy the theme you can use it only on a single site.

What if i use it first on one site and after few months can i use it on another site ?

You can only use it on one site at any given time per license. If you remove the theme from your website then you can use the theme on a different website. But only one website can have the theme activated at a single time.