Norway is long and diverse. The climate varies a lot from fiords to mountians. From south to north. Www.yr.no will give you Some answers.
Generally- south can vary from more than 20 celsius at the warmest. Cool 12 degrees on the cold side in the summer. Norway has cold winters. They cen be wet in the coast or snowy, white in the north and in the eastern parts of Norway.
Appropiate time depends on what you want to see. All seasons have their own charm. My favourite is spring. Welcome to Norway. Have fun. almost 6 years ago