for the first year it is around 20-30%.
later it all depends on how many new nodes will be added ofc

this is a handy tool for monthly profit estimation (it loads current number of active nodes automatically):
https://zednode.com/znode-stats.html

also this image gives a fair idea:
http://zcoin.io/wp-content/uploads/2017/11/ZCOINMN_V3.png

happy new year