ISS is visible after dusk or just before dawn when the light still exists 200 miles up but darkness is still below. The number of hours this happens is longer in the summer meaning sometimes the ISS can be viewed twice a night.
Viewings are either only at night, only at dawn, or in a period of maybe two weeks with no sighting opportunities.
ISS is about as bright as a lighted airplane. It takes about 3 minutes to cross the sky. It can be viewed anywhere below on the earth in a circle with radius of maybe less than 300 miles.
Your opportunities are provided at:
www.heavens-above.com or NASA's
human space flight server.
The ISS is now so large that some have reported seeing it in daylight with clearest skys.