When can we expect next airdrop?
and is it true that rewards are getting reduced

What you receive, is proportional to your balances in BTC and Bytes on the Full Moon of November, on November 4, 2017 at 05:23 UTC:
BTC to bytes: 1 BTC of proven balance gives you 6.25 MB (0.00625 GB)
BTC to blackbytes: 1 BTC of proven balance gives you 2.1111 * 6.25 million blackbytes (money supply of blackbytes is 2.1111 times more than that of bytes)
Bytes to bytes: 1 byte on any Byteball address gives you 0.1 new bytes
Bytes to blackbytes: 1 byte on linked Byteball address gives you 0.21111 blackbytes