@johnn_le: I posted the almost same question on reddit and got following answer:

In 300 days you should make back your 1000SLM, in 1.5-1.8 year, you should make about 1.7x-2x of your investment, but it depends on several factors (difficulty of network is the most important) We are actually working on a calculator, which will include several factors into calculations.

https://www.reddit.com/r/slimcoin/comments/7la49c/now_slimcoin_community_account_is_here/