Board Thread:ReLIFE Manga Discussion/@comment-107.167.108.171-20170706064942/@comment-32743732-20170726230735

TL;DR: 2 and 1/2 months.

Explanation: If you check the first chapter of ReLIFE, you'll see that Ryo Yoake made his first contact to Arata Kaizaki on March 1. This is safe to assume that most high schools in Japan end at the first two weeks of Spring which happens to land in March. For the time being, in Chapter 188, Ryo Yoake made another report dated December 16. Therefore, using simple math, we get more or less 2 and a half months remaining!~