Interval really depends on the subject of your time-lapse and how long the resulting movie should be. Sunrise/sunset I like around 4 to 6 seconds interval, fast moving clouds around 2 seconds. If you wanna do astro time-lapse you need long exposures anyhow for correct exposure, unless you have a super fast lens and can go for really high iso. For astro an interval of 15 seconds or more is common I think, so you can expose for at least 12 seconds. Stars don't move very fast (from our point of view), so an interval of 1,5 seconds would result in very little movement in your time-lapse, unless you render at super high framerates. But that would be a silly thing to do.
Sadly I don't have much experience with astro timelapses though, so much light pollution in my country.